Encrypted history sharing: when I am in a room for which I have received details of a key bundle, trigger a download and import

[richvdh]

Our proposed solution to sharing encrypted room history on invite involves sharing the keys to an encrypted blob of room keys via to-device messages.

#4512 tracks receiving the to-device messages. #4513 tracks an implementation of downloading and importing the keys. This issue tracks tying the two together.

Note that to-device messages can be delayed, so it is possible that we receive the to-device message after accepting the invite. We need to handle this scenario as well as the more normal case that the to-device message was received first.