Support re-sending e2e keys to other participants in the room

[Biep]

Is your suggestion related to a problem? Please describe.
When I still have access to my old e2ee messages, but my room mate hasn't, I want to provide those messages again.

This is especially acute now that many people have lost their keys due to the measures taken after the incident.

Describe the solution you'd like
If one member of an encrypted room has lost the keys, but another member still has them, allow that other member to resend all keys/messages to the first member. If needed a cut-off for messages from before the time of joining could be applied. This should 'revive' the currently unreadable messages rather than send copies.

[aaronraimist]

Looks like you opened this twice. I'll leave the newer one open since it has some more discussion.

Duplicate of #12275

Also related: https://github.com/vector-im/riot-web/issues/2286

[Biep]

Early onset Alzheimer kicking in again..