Garbage collection of attachment files once a room is left

[leijurv]

Feature request: After leaving a room, I would like a way (maybe an option) to clean up all local data (on my synapse homeserver) that came from that room, such as attachments.

I have gigabytes of attachments from a room that I unthinkingly stayed in for a few months, and I don't know if I can safely delete them without messing up synapse.

[ShadowJonathan]

Is it remote media? Then you can just delete the remote media folder.

[erikjohnston]

We should only do this automatically if all users on the server "forget" the room.

[ShadowJonathan]

I think that's a bad idea, because some users might've forwarded messages from that room to other rooms, and/or used media for other purposes, which then becomes irretrievable after we clean them from a room.

[richvdh]

related #4720

[richvdh]

related: #1263