Room garbage collection

[pmaier1]

As an admin I want that all content of a room is deleted when a room is removed for that there is no orphaned events/messages/media and I can keep storage under control as well as comply with legal requirements.

• Deleting a room should delete all of its contents (events/media)
• This should work for local rooms as well as remote rooms that comply with the spec

[clokep]

@pmaier1 Does this differ from #4720?

[DMRobertson]

I don't think media is delete when a room is purged?

(Because the same MXC may be reused in another room? Though I spoke to @kegsay last week who was of the opinion that this is a bug in Synapse.)

[clokep]

who was of the opinion that this is a bug in Synapse.

It isn't a bug, it is part of the spec that MXCs can be re-used. I think some implementations (matrix-media-repo?) will issue new MXCs without duplicating the backing files though, which seems quite nice...but doesn't help in encrypted rooms.