A lot of the functions have the same name in this space like `store_file`,
and we also do it multiple times for different reasons (main media repo,
other storage providers, thumbnails, etc) so it's good to differentiate
them so your head doesn't explode.
Follow-up to https://github.com/matrix-org/synapse/pull/15850
Tracing instrumentation to media `/upload` code paths to investigate https://github.com/matrix-org/synapse/issues/15841
* 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.
Eric Eastwood