Invisible crypto: allow the user to review recent changes to other users' identities

[richvdh]

Part of #2491, itself part of Invisible crypto.

If a user dismisses an identity change banner (cf #2492) too quickly, they may wish to review the list of recently-seen changes.

[richvdh]

XXX Questions:

• How far back does the history need to go? Is “since last client restart” sufficient?
• If a user rotates their identity twice, should they appear twice on the history list? It might be indicative of something fishy?
• Should this be per-room, or global? If it’s per-room, should users who only changed their identity before they joined the room, or since they left the room, be included?
• We don't yet have designs for this.

[richvdh]

We've agreed we don't need this, for now at least.