create api_client class to call backend, partial bot rewrite

[andreaskoepf]

Basic bot communication works, but we need to decide how the bot-user interaction really works.
Currently I have implemented a dummy time base protocol which sends messages in a bot channel.
Maybe it is better to go with commands to initialize actions.

But then we still need an intuitive way to interact with the bot.

[yk]

LGTM, thank you!

Some things to improve (but can be done later):

• use httpx instead of requests for async i/o
• logging instead of prints
• general cleanup, including sharing code between bot and backend (I'll take that)