This change is part of an ongoing, aggressive effort to clean up the
codebase and add support for sharing Android binaries, so a couple of
changes included here are slightly out-of-scope. Most of them concern
the dialogue options themselves.

- Fix type hints for callback queries
- Improve order of imports using isort
- Rename "OnionSproutsBot" Client instance to "Bot" to save columns and
  improve clarity
- Apply black's default formatting settings to all source files
- Small string improvements
- Improve comments
- Add type hints
- Fix incorrect type hints
- Fix problem where "Main Menu" button would not pop up when asking for
  mirrors.
- Use separate function for error messages
- Error messages now appear only once and are always descriptive
- Use language parameter when possible

Co-authored-by: Alain Zscheile <zseri.devel@ytrizja.de>
Co-authored-by: StarByte <OpenSource@stacksmash.net>