Tidy up code and split into multiple files.

[GoodClover]

The current code is a monolithic one-file mess.

At some point the following should probably be done:

• [ ] A bunch of constants are defined in main.py that should probably be moved to the config.
• [ ] Split everything into multiple files so it is better organised. Not sure how they work but I think this is what Cogs are for, slash-command supports them.
• [ ] Clean the type annotations up, some are missing, some wrong. It doesn't help that discord.py doesn't have them (slash-command I think does).
• [ ] Better code comments.

[kallejre]

Another one:

• [ ] Merge COMMANDS.md and HELP.md into help file, update readme to reflect changes.
• [ ] Unify slash command and on_message inline element handling code.
• [ ] Convert unit testing experiment to something more useful.
• [ ] (Considering your Pi has probably multiple cores, maybe offload some resource-intensive tasks to other python processes. They can be linked using http connections within localhost (e.g ports :123546 and :123465).)
• [ ] Sub-suggestion. Since you had networking issues with tile server, perhaps add something like proxy.py, that would handle load balancing and caching when querying external resources?
• [ ] The new fancy help UI with buttons could be used to create Discord's OSM editor 😄