refactor: clean up __init__ file following some Flask conventions

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")

tests/conftest.py

@@ -1,10 +1,11 @@
 import pytest
 
-from lnbits import app
+from lnbits.app import create_app
 
 
 @pytest.fixture
 def client():
+    app = create_app()
     app.config["TESTING"] = True
 
     with app.test_client() as client: