✨(frontend) allow promoting authenticated participants

Introduce a new feature that lets a user promote one of the
authenticated participants of the meeting to a role with additional
privileges.

Known limitations:

* Only authenticated participants can be promoted, but there is no
  visual indicator yet distinguishing authenticated from anonymous
  participants. This will be added in a follow-up commit.
* The resource_access data fetched in the initial API call becomes
  stale after a promotion. It is not currently used in the product,
  so this is not visible, but it should either be refreshed later
  or removed from the initial fetch.
* Demoting a promoted user turns them into a member, which is still
  a privileged role. This is a deliberate choice until we introduce
  finer-grained tuning of participant roles.

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to
 
 - ✨(summary) report exception type in failure analytics
 - ✨(frontend) add configurable documentation menu item
+- ✨(frontend) allow promoting authenticated participants
 
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