Garbage collection of attachment files once a room is left

This issue has been migrated from [#10176](https://github.com/matrix-org/synapse/issues/10176).

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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.