Improve the E2EE device verification and keys requesting flow

[Palid]

Currently the flow to request keys to unencrypt old messages on a new device is extremely confusing for a lot of users. We don't provide enough data to the users that they will be able to unecrypt only messages from cross-verified sessions, and not from all the verified sessions.

There's also the case where users click "request keys" link under a message, but that doesn't do anything because this session is not cross-verified with a session that has those keys.

The general idea about this issue is to:

• Improve copy for those actions to properly notify user that they may not be able to unencrypt messages if they aren't cross-verified with proper session
• Add some big, error modal window that your sessions are not properly verified, as of above, and that the keys request has failed

Probably adding a way to see which sessions are cross-verified in the sessions list would be quite useful too, but if I remember correctly we don't exactly have a way of knowing this?

Related: https://github.com/vector-im/element-web/issues/18652

[americanrefugee]

Hello! This seems related to Issue #9544, in which case perhaps that will fix things. If not, then we can pick this back up whenever it makes sense. Either way I'm happy to support on this issue.