Replace Dashboard Activities by Vue Components

What does this MR do?

• Refactors Frontend and Backend code for dashboard.
  Before: HTML and JS data was created in php & sent to activities.js).
  Now: All HTML and JS moved to vue.js files, the backend only sends data
  Backend: fetching updates is structured and replaces the previously different load+loadMore with pagination support
• Infinite scrolling to automatically load new data was also moved to vue.js
• Added Dashboard filtering options by types (store updates, mailbox messages, event posts etc)
  new update types: foodsharepoint wall posts (addressing part of #588 (closed)) and event wall posts. Closes #227 (closed)
• Some text was made translatable and moved to the client where possible
• Quickreply support for (in theory) all update types
  (we should figure out which of them make sense to offer)
• Image thumbnail display for wallpost snippets on the dashboard!
  Available for profile/event/foodsharepoint posts. Closes #454 (closed)
• Allow having many disabled update sources: closes #365 (closed)
• Fix cut-off partial links in dashboard snippets: closes #691 (closed)
• Closes #849 (closed) broken date on forum posts. This is marked as release blocking in #793 (closed)

How confident are you it won't break things if deployed?

Definitely needs review. Changed activities xhr app to not return html anymore, rebuilt dashboard completely...

Screenshots

Finished

• Readd settings
• Readd markdown
• Readd quickreply functions
• Readd timeago / from-now
• Readd lazy loading
• User pictures on dashboard are links now, closes #464 (closed)
• Add thumbnails of pictures on friend wallposts
• Add thumbnails to other wallposts too
• Event updates on dashboard
• FoodSharePoint updates on dashboard- [x] send no-more-data thingy to frontend ( ❓ does that fix the Endless loading indicator below)
• give nicer message when No more data can be loaded
• Trigger reload to check for new data when switching between filter types
• CSS fixes to make sure the filter bar height doesn't jump => @ChrisOelmueller knows more

Nice to have or ToDos after merge

• Add tests
• Add better seed data
• Differentiate between listings & activities ❓
• Remove mail symbol for non-mail users. Method mayHaveMailbox() can be used here => @jofranz knows more

Bugs and crucial ToDos preventing a merge

• Endless loading indicator: (@jofranz knows more)
  If a section is empty and you scroll down to refresh, there is an endless loading indicator
  Around three requests per second are sent with pages of pagination counting up.
  Solution idea: ❓
• sort FSP updates by date
• on dashboard clicking on a event image links the userprofile, instead of the event page

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))