Quotas for file and event storage

This issue has been migrated from [#3339](https://github.com/matrix-org/synapse/issues/3339).

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Something that keeps coming up is the need to track which users are responsible for chewing all the storage on your server - whether that's by storing lots of stuff in the media repo, or by joining massive rooms and chewing all your DB (or submitting loads of events).

We probably need to distinguish storage which the user created directly (i.e. by sending a file or event) or indirectly (by accessing a file or a room).

We probably care more about directly attributable storage costs, as indirect ones are going to be shared across a bunch of users - plus tracking the indirect ones could result in a metadata leak for e2e rooms (i.e. which users have access to the files in a given room)