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.
- extensions are now blueprints: keep views, templastes and statics in the same folder
- increase app security using `flask-talisman`
- whenever possible use {{ url_for }} for links between pages
- remove references to non-existing JavaScript code
- add missing favicon.ico
