Invisible crypto: inform users about changes to other users' cryptographic identity

[richvdh]

As part of the Trust and decorations work, we want to show users when somebody in a room changes their identity (ie, Master cross-signing key).

There are ~~three~~ ~~two~~ ~~three~~ four parts to this work:

1. [x] #2492
2. [ ] #2493
3. [x] https://github.com/element-hq/element-meta/issues/2544 a. [ ] element-hq/element-meta/issues/2526 c. [ ] element-hq/element-meta/issues/2525 d. [ ] element-hq/element-meta/issues/2524
4. [x] #2513
5. [x] https://github.com/element-hq/element-web/issues/27943
6. [x] ~~#2494~~

[richvdh]

Notes from a design meeting today:

• There is probably no need for a warning banner (#2492) when an unverified user replaces their identity -- in other words, as soon as we see the new identity we replace it.
• The in-timeline notice for identity changes (#2493) could do with being more emphasised than just a nick change, but not highlighted in red.
• Do we want the same timeline notices for verified and unverified users? Probably the same styling but slightly different wording.
• What happens if there are multiple verified users with new identities? For now, we probably just queue them up, but eventually maybe we want to expose a modal where you can review the affected users, or something.
• Do we need the "Verify again" button?
  • Most of the time you won't be in a position to re-verify, so it's a bit useless
  • but we need something other than just "withdraw verification"
  • maybe it should be "go to profile"?

[richvdh]

More notes:

• #2493 is involved. In the meantime we'll have to display a warning banner for unverified users: #2513 covers this