Prior to this PR, remote downloads which did not provide a
`content-length` were decremented from the remote download ratelimiter
at the max allowable size, leading to excessive ratelimiting - see
https://github.com/element-hq/synapse/issues/17394.
This PR adds a linearizer to limit concurrent remote downloads to 6 per
IP address, and decrements remote downloads without a `content-length`
from the ratelimiter *after* the download is complete and the response
length is known.
Also adds logic to ensure that responses with a known length respect the
`max_download_size`.
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
Implement MSC3860 to follow redirects for federated media downloads.
Note that the Client-Server API doesn't support this (yet) since the media
repository in Synapse doesn't have a way of supporting redirects.
This converts the media servlet URLs in the same way as
(most) of the rest of Synapse. This will give more flexibility
in the versions each endpoint exists under.
The stubs have some issues so this has some generous cast
and ignores in it, but it is better than not having stubs.
Note that confusing that Element is a function which creates
_Element instances (and similarly for Comment).
There are two situations which were previously not properly checked:
1. If the requested URL was replaced with an oEmbed URL, then the
oEmbed URL was not checked against url_preview_url_blacklist.
2. Follow-up URLs (either via autodiscovery of oEmbed or to pre-cache
images) were not checked against url_preview_url_blacklist.
* 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.
