Telegram content proxy

[ForNeVeR]

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

TODO:

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