Delete media from the server when the event is redacted

In theory if we do this right on the technical side then there won't be any client changes, however this is an important enough feature to need to point people at UI-wise. We might also want to add a checkbox or similar for "[ ] and all related media", so could argue it needs design 😇

This issue also doubles as a launching point if product/design wants to find a way to push it forward (alongside all other features and responsibilities, of course).

Technical links:
* https://github.com/matrix-org/synapse/issues/1263
* https://github.com/matrix-org/matrix-spec/issues/226
* Links within the above to MSCs and other spec/impl issues

Start by reading Synapse issue #1263 and Matrix spec issue #226, then follow the MSCs and implementation issues linked from them. Done requires an agreed technical and product/design direction for handling media when an event is redacted; this issue names no files or tests to run.

Written by the indexing model from the issue text.