Check that media repository times out correctly when downloading infinite media sources

[ara4n]

In theory we have a maximum file size limit when downloading, i /think/, but need to check that it's being upheld correctly for things like https://hookrace.net/time.gif

[ara4n]

This is particularly unpleasant if someone pastes a live HTTP stream into a room for obvious reasons - thanks to href for pointing this out.

[clokep]

I think #8302 ended up being a duplicate of this, which was fixed by #11784, hopefully.