This stops media (and thumbnails) from being accessed from the
listed domains. It does not delete any already locally cached media,
but will prevent accessing it.
Note that admin APIs are unaffected by this change.
* Removes the `v1` directory from `test.rest.media.v1`.
* Moves the non-REST code from `synapse.rest.media.v1` to `synapse.media`.
* Flatten the `v1` directory from `synapse.rest.media`, but leave compatiblity
with 3rd party media repositories and spam checkers.