Handle pre-shared invite keys

[AndrewFerr]

Signed-off-by: Andrew Ferrazzutti andrewf@element.io

Checklist

• [ ] Tests written for all new code
• [x] Linter has been satisfied
• [x] Sign-off given on the changes (see CONTRIBUTING.md)

[AndrewFerr]

the other thing I'm not sure on is whether this is actually recommended by the spec/rust-sdk - it doesn't appear to come up in documentation, though I'm willing to admit I might be blind.

FWIW this is the behaviour Element Web uses. If you've invited a user with an encryption-enabled device to an encrypted room, EW will send the room key to that user (via a to-device message) even before they've joined the room. In fact, it sends the key even before you post a message -- it sends it as soon as you starting typing a message!