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")

lnbits/app.py

@@ -0,0 +1,75 @@
+import importlib
+
+from flask import Flask
+from flask_assets import Bundle  # type: ignore
+from flask_cors import CORS  # type: ignore
+from flask_talisman import Talisman  # type: ignore
+from werkzeug.middleware.proxy_fix import ProxyFix
+
+from .commands import legacy_migrate
+from .core import core_app
+from .ext import assets, compress
+from .helpers import get_valid_extensions
+
+
+def create_app(config_object="lnbits.settings") -> Flask:
+    """Create application factory, as explained here: http://flask.pocoo.org/docs/patterns/appfactories/.
+    :param config_object: The configuration object to use.
+    """
+    app = Flask(__name__, static_folder="static")
+    app.wsgi_app = ProxyFix(app.wsgi_app, x_proto=1, x_host=1)  # type: ignore
+    app.config.from_object(config_object)
+
+    register_flask_extensions(app)
+    register_blueprints(app)
+    register_filters(app)
+    register_commands(app)
+
+    return app
+
+
+def register_blueprints(app) -> None:
+    """Register Flask blueprints / LNbits extensions."""
+    app.register_blueprint(core_app)
+
+    for ext in get_valid_extensions():
+        try:
+            ext_module = importlib.import_module(f"lnbits.extensions.{ext.code}")
+            app.register_blueprint(getattr(ext_module, f"{ext.code}_ext"), url_prefix=f"/{ext.code}")
+        except Exception:
+            raise ImportError(f"Please make sure that the extension `{ext.code}` follows conventions.")
+
+
+def register_commands(app):
+    """Register Click commands."""
+    app.cli.add_command(legacy_migrate)
+
+
+def register_flask_extensions(app):
+    """Register Flask extensions."""
+    """If possible we use the .init_app() option so that Blueprints can also use extensions."""
+    CORS(app)
+    Talisman(
+        app,
+        force_https=app.config["FORCE_HTTPS"],
+        content_security_policy={
+            "default-src": [
+                "'self'",
+                "'unsafe-eval'",
+                "'unsafe-inline'",
+                "blob:",
+                "api.opennode.co",
+            ]
+        },
+    )
+
+    assets.init_app(app)
+    assets.register("base_css", Bundle("scss/base.scss", filters="pyscss", output="css/base.css"))
+    compress.init_app(app)
+
+
+def register_filters(app):
+    """Jinja filters."""
+    app.jinja_env.globals["DEBUG"] = app.config["DEBUG"]
+    app.jinja_env.globals["EXTENSIONS"] = get_valid_extensions()
+    app.jinja_env.globals["SITE_TITLE"] = app.config["LNBITS_SITE_TITLE"]