a big refactor that:
- fixes some issues that might have happened (or not) with asynchronous
reactions to payments;
- paves the way to https://github.com/lnbits/lnbits/issues/121;
- uses more async/await notation which just looks nice; and
- makes it simple(r?) for one extension to modify stuff from other extensions.
Flask extensions are loaded in a way that makes them easily reusable by blueprints.
In this commit we are also adding `environs` to manage .env and settings:
breaking changes!
- FLASK_APP=lnbits.app
- LNBITS_ALLOWED_USERS needs to be empty now to allow all users (NOT "all")
