Reimplement mailbox list in Vue

What does this MR do?

• Reimplement mailbox list in Vue
  • Removes the annoying dynatree usage
  • Display (static, server-fetched) unread badge
  • "Expand" all possible mailboxes, to reduce amount of clicks needed
  • Consistent background loading when using show parameter
  • Remove options/actions menu in favor of a simple green button (Refresh is gone for now)
  • Remove unused (as far as I could tell) mailbox-type tracking
• Adjust a gateway method for easier understanding, this should be performance-checked!
• Fix problem with ancient Xhr typeahead (and another bug in mailbox management)
• Fix a bug with v_field title span
• Standardize remaining v_form_tagselect calls

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

I tried to keep as much of the ancient code surrounding the folder list intact, to reduce the possible breakage. during local testing and with local test data, things seem to work. still... our live mail data is rather broken, so this will need lots of beta-testing before it's ready.

Links to related issues

• &23

How to test

1. Checkout branch locally
2. Login as bot
3. Access mailboxes and messages (e.g. from dashboard update link, from topbar menu link) + click around
4. Login as orga
5. Manage mailboxes: create a new one and assign users to it (this should work again now)

Screenshots

Checklist

• mostly UI changes, no valuable test functionality for those parts
• no unrelated changes there is some legacy-viewcode refactoring in here, triggered by the changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a release notes snippet
• I am unable to take care of the beta-testing task

Release notes text

see release-notes/2021-03/1788.md