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.
