Communities v2 prototype: "In community" view

[turt2live]

Based on #5153 - see pull/5153/head...pull/5161/head for real diff

Reviewer: In addition to all the prototype behaviour described below, this includes a few cleanups to the prototype:

• Use the more reliable FlairStore for profile information (apparently GroupStore is bad at this).
• Move several prototype-specific behaviours and functions into the prototype store for handling.
• Makes the ... button on the UserMenu a chevron - this is intentionally done outside the prototype by design direction.

It may be best to review this commit-by-commit, though it's not my cleanest PR ever (sorry).

Prototype behaviour:

• When viewing a community, use the UserMenu to indicate which one
  • Rationale: The user can easily get confused as to where they are, so try to minimize that.
• When not viewing a community, clearly indicate that they are at the "Home" view
  • Rationale: Avoid text-jarring behaviour when switching to/from a community by treating Home as a community.
• When viewing a community, the user menu's menu options represent both the user's and the community's
  • Rationale: We're not sure what the user will try and do here, so present them with a menu for the community and their own account.
  • This new menu is only used when there's a community selected and the user is in the prototype. The larger version is untouched when the user is at the Home view.
  • The menu is intentionally condensed to try and have roughly the same height as the Home one, which is nearly at the maximum size we're willing to accommodate. As such, some options have gone missing from the user's half.

Viewing community:

Viewing Home:

Edit dialog:

[turt2live]

Builds failure is due to a Synapse issue

[jryans]

Hmm, is it intentional to change this for non-prototype users as well?

[turt2live]

Yes: