When we redact events, any mxc content they refer to should be redacted too (SYN-216)

[matrixbot]

This issue has been migrated from #1263.

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It's a bit of a disasterous thinko that we can redact events which point to stuff in the media repo, and that content is subsequently preserved even though the event is nuked. We should rm it and its caches too (assuming the HS is honouring redactions).

(Imported from https://matrix.org/jira/browse/SYN-216)

(Reported by @ara4n)