Telegram content proxy

[ForNeVeR]

Closes #102. I've decided we need no statistics aside from logs.

TODO:

• implement the file cache
• cache cleanup: ensure there's a sanity check (to not accidentally clean up an unrelated directory)
• squash WIP commits
• don't forget file cache tests!
• make sure Emulsion works in the old way (t.me proxies) with the file cache disabled
• tests for all the branches in the FileCache
• tests for all the branches in the ContentController
• make all the tests green
• make sure there are no TODO remaining
• content types
• file names for download
• manual testing
• CI: investigate the test failures on Windows
• deployment instructions, Docker upgrade
• review everything

[gsomix]

Probably leak.

[ForNeVeR]

We have several places with while true in async code already, and they seem non-problematic for now. I think I'll leave that as-is, but will keep an eye on it.