Implement MSC2432: Alias semantics

[richvdh]

We've got yet another idea for how to implement alias semantics, documented in https://docs.google.com/document/d/1NNDkobiFLeUkJtyj0H6qvKIedgvIkZnFKo78-03cGEk and MSC2432.

Here are the things I think need doing in Riot (et al):

• stop formatting m.room.aliases events specially in timelines
• stop considering m.room.aliases events for room names
• Update Room Settings UI to:
  • remove display of m.room.aliases, since they are now meaningless
  • let users see all the aliases for the room on the local server
  • hide local aliases behind an ‘advanced’ disclosure or something, just to help hide obnoxious local ones by default unless you’re really hunting for them
  • let users see the current "main" and "alternative" aliases from the m.room.canonical_aliases list
  • let mods twiddle the list of advertised aliases and send updated canonical_aliases events

[richvdh]

The synapse side of this is matrix-org/synapse#6898.

[jryans]

We'll start with updating the local aliases as a subset of this in #12408.

[bwindels]

Already started working on editing alt_aliases on branch bwindels/altaliases fwiw. Parked it now as it is not priority.

[bwindels]

PRs are out for all of the above checkboxes, but doing a search for m.room.aliases in the code shows still quite a few occurrences. Need to go back to cross-signing now, but we might want to clean that up at some point.

[bwindels]

Filed #12680 to track the remaining cleanup, closing this.