Add room selection to E2EE key export

[sancarn]

We've had issues recently where clients have been unable to receive E2EE keys. Generally speaking this shouldn't happen but in the case that it does a simple work around is exporting your E2EE keys to a text file, sending the text file to the other person, and they can import your keys into their affected client.

The problem however is in doing so you share ALL your room keys, which includes all DM rooms and all other server keys.

It would be nice if you could select which rooms specifically you want to export the keys for, so you can better control what keys are stored in your E2EE key backup. Either this or the ability to easily edit MEGOLM files - however I feel this would be a bigger task.

[aaronraimist]

Duplicate of element-hq/element-meta#1287