Add option to start a new resolvable discussion in an MR

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/24378

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1556

To do:

• Specs
• Reply by email support
• Better notifications (https://gitlab.com/gitlab-org/gitlab-ce/issues/24378#note_25365790)
• Consider the case where a new diff comment can become separated of an existing discussion (comment created while existing discussion was not visible, for example before it was loaded - uncommon, or when its position was out of date - unlikely)
• Remove/ignore the notes.original_discussion_id column
• Documentation https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10390

Out of scope:

• API support (https://gitlab.com/gitlab-org/gitlab-ce/issues/20901)

Button in form:

After tabbing to the button:

After hitting Command+Option+Return (or Ctrl-Alt-Enter on Windows), a new discussion appears:

/cc @smcgivern