Let megolm session keys be available to devices added by invited users since the point they are invited

[ara4n]

Closely related to matrix-org/matrix-spec-proposals#2286 - when we invite someone into a room we should give them enough megolm session data to let them decrypt the messages that they should be able to view before joining (if any).

[ara4n]

Hopefully fixed by matrix-org/matrix-js-sdk#666, matrix-org/matrix-react-sdk#2042 and matrix-org/synapse#3484.