Issue discussions Vue refactor

This MR is about to refactor Issue discussions with Vue to allow us to build new features onto it, like starting a new thread from a non-discussion comment.

Related with #30299 (closed) and !11687 (closed)

/cc @DouweM

List of things to do as discussed with @jschatz1 in this MR

• You can't stop the polling... For when you change tabs. Can't use the current polling function because VueX.
• When you poll you lose the emojis.
• [ ] After a Slash command the state is not updating. (This also doesn't happen on master or gitlab.com, waiting on @DouweM feedback on this) issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/35929
• Using a Vue Instance to pass events to jQuery code. Remove the Vue instance to stop providing events.
• If you close the issue in issue buttons. If you do something in the sidebar. Nothing updated.
• When you leave the page the reply should still be there.
• Make sure we show first class action errors properly.

List of bugs @iamphill found:

• Alignment seems all wrong
• JS error when saving edit on note:
• Can't reply on a discussion
• [ ] I wonder if we should trigger a poll when we close/re-open the issue or apply slash commands issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/35929
• I'm not sure how I did it, but I managed to get the issue state as closed but the Close issue button was still there: issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/35929
• Toggle discussion doesn't work
• Added a award emoji to a single note got an error but the emoji stayed there even after a refresh
• Register/Sign-in link lead to nowhere
• Submit button doesn't disable when submitting a comment
• Linking to a comment doesn't work
• Slash commands should be disabled on edit comments
• It is possible to submit an empty reply to a discussion

List of things @jschatz1 said to not fix in this MR:

• We are updating using the window.gl obj and updating that obj through vue.
• No tests for the VueX part.
• No tests for the Vue components

### Frontend checklist

• Render single note
• Render discussion
  • Implement toggle control for discussions
  • Show register and sign in links in discussion footer for anon user
• Render system notes
• First class note actions
  • Handle note edit
  • Handle note delete
  • Handle discussion reply
  • Handle new comment created
  • Handle new discussion created
• First class issue actions
  • Handle issue close
  • Handle comment and close
  • Handle start discussion and close
  • Handle issue reopen
  • Handle comment and reopen
  • Handle start discussion and reopen
  • Make sure that we updating the issue badge after closing/reopening the issue
• Award Emoji
  • Create award emoji list
  • Handle adding new emoji from emoji dropdown
  • Handle adding new emoji from emoji list
  • Handle emoji click in emoji list
  • Highlight already awarded emoji
  • Handle mutual exclusive emojis
  • Render and at the beginning of the list
  • Disable adding and to your own comment
• Realtime
  • Implement polling for changes
  • Handle response and render new notes/discussions
  • Refactor polling mechanism to use next generation polling
• Known issues, improvements, things to do later
  • Fix syntax highlighting
  • Fix KaTex rendering
  • Fix to do list editability
  • Make sure that we create dummy note placeholders while requesting
  • Make sure that we implement Slash commands and autocompletes properly
  • Make sure that up arrow puts last comment into edit mode
  • Create empty comment widget with register and sign in links if user has no permission
  • Remove award emoji related vendored svg's and use them from icons folder
  • Timestamp needs to link to #note_<id>
  • When anchor is #note_<id>, we need to jump to that note
  • Scroll the anchored note into view after notes rendered if page has note anchor
  • "Add reaction" needs to be hidden when signed out
  • Options menu and "Report as abuse" need to be hidden when signed out
  • "Report as abuse" should not be shown for comments by current user when signed in
  • "Preview" pane should say "Nothing to preview" when textarea is empty
  • "Comment" button should be disabled when textarea is empty
  • New notes and note edits don't have preview
  • "Toggle discussion" arrow doesn't flip
  • ESC doesn't cancel a note edit or discussion reply (immediately cancels when textarea is empty, and shows confirmation alert otherwise)
  • Show register and sign in links instead of comment when signed out
  • Fix autocomplete bug, request returns HTML and it causes recursive errors on the page
  • Fix slash command only comment
  • Fix syntax highlighting after editing a comment
  • When issue is closed/reopened from the issue actions at the top of the page, it should properly update close/reopen button state on the main note form at the bottom of the page
  • Handle quick actions notes.js:285
  • Fix visual bugs new note form, note type dropdown
  • Fix resurrecting autosaved comments into the reply widget
  • Make sure that we show first class action errors properly
  • Bottom padding of discussion reply "Comment" button is inconsistent with MR page
  • Discussion reply form should say "Markdown and quick actions are supported", not just "Markdown is supported" (only when adding a reply, not when editing)
  • When issue state changed using a slash command, we don't update the issue state buttons on the new note form
  • emoji bug
  • Polling removes awards
  • Zen mode editing shows comment toolbar SS
  • Keyboard shortcuts
• Technical Debts - [ ] All components starts with issue_note prefix and all of them under notes/components folder. Most of them should be moved into shared components folder and issue_ prefix should be removed
  • Use constants in Vuex
  • Separate state, getters, actions, mutations in Vuex store to their own files