Loudly warn if you are DMing a user who has no E2EE capable devices

[ara4n]

Currently if you DM a user, your client will dutifully auto-enable E2EE - but if that user doesn't yet exist (it's a 3PID invite as per #12902), or if we've yet to sync their device list and get their E2EE keys, we will almost-silently send unencrypted messages to them (as per #12313).

Instead, we should put an explicit big warning in the timeline that we haven't found any encryption capable devices for this user yet, and so any messages sent will not be E2EE.