Import history when creating bridged room

[remram44]

Right now the room is created empty, although Gitter supports retrieving old messages and Matrix supports injecting them.

Importing the full history is not a good idea, but a limited number of recent messages would be useful.

[roblabla]

Isn't there a way to make the matrix server query the appservice for "older messages" ? That seems like a useful addition.

Something like the backfill from Server-Server API (https://matrix.org/docs/spec/server_server/unstable.html#backfilling)

[remram44]

You can't add older messages to the Matrix history, because of the way the history is linked together in a temper-resistant way (with hashes, like the blockchain). So a Matrix server can get more of the Matrix history for a room lazily, but that history has to already exist and be included in the hashes of more recent messages.