Fixes up #17239
We need to keep the spam check within the `try/except` block. Also makes
it so that we don't enter the top span twice.
Also also ensures that we get the right thumbnail length.
When a module rejects a piece of media we end up trying to close the
same logging context twice.
Instead of fixing the existing code we refactor to use an async context
manager, which is easier to write correctly.
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
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.
