Require acknowledgement to send to a verified user if their identity changed or a device is unverified

[richvdh]

Recently we have done some work on Element X to ensure that we show an error if the user sends an encrypted message and:

• A verified person becomes unverified, or:
• A verified person has a new unsigned device

(This work was tracked at element-hq/element-meta#2500 and element-hq/element-meta#2501)

I believe we also need to implement this on Element Web.

[richvdh]

Possibly matrix-org/matrix-rust-sdk#3662 does most of the groundwork for this but we need to actually wire it up in web and make sure it works

[andybalaam]

Moving to unprioritised backlog because once element-hq/element-meta#2492 is done, this will be very rare, and it is not a security problem - just a misleading error message when the message fails to send.

Note: ideally, instead of showing an error message, we would pick up the draft message and display it in the composer, with the warning banner already visible.