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/__init__.py

@@ -1,116 +0,0 @@
-import re
-import importlib
-import sqlite3
-
-from flask import Flask
-from flask_assets import Environment, Bundle  # type: ignore
-from flask_compress import Compress  # type: ignore
-from flask_cors import CORS  # type: ignore
-from flask_talisman import Talisman  # type: ignore
-from os import getenv
-from werkzeug.middleware.proxy_fix import ProxyFix
-
-from .core import core_app
-from .helpers import ExtensionManager
-from .settings import FORCE_HTTPS
-from .db import open_db, open_ext_db
-
-
-disabled_extensions = getenv("LNBITS_DISABLED_EXTENSIONS", "").split(",")
-
-app = Flask(__name__)
-app.wsgi_app = ProxyFix(app.wsgi_app, x_proto=1, x_host=1)  # type: ignore
-valid_extensions = [ext for ext in ExtensionManager(disabled=disabled_extensions).extensions if ext.is_valid]
-
-
-# optimization & security
-# -----------------------
-
-Compress(app)
-CORS(app)
-Talisman(
-    app,
-    force_https=FORCE_HTTPS,
-    content_security_policy={
-        "default-src": [
-            "'self'",
-            "'unsafe-eval'",
-            "'unsafe-inline'",
-            "blob:",
-            "api.opennode.co",
-        ]
-    },
-)
-
-
-# blueprints / extensions
-# -----------------------
-
-app.register_blueprint(core_app)
-
-for ext in 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.")
-
-
-# filters
-# -------
-
-app.jinja_env.globals["DEBUG"] = app.config["DEBUG"]
-app.jinja_env.globals["EXTENSIONS"] = valid_extensions
-app.jinja_env.globals["SITE_TITLE"] = getenv("LNBITS_SITE_TITLE", "LNbits")
-
-
-# assets
-# ------
-
-assets = Environment(app)
-assets.url = app.static_url_path
-assets.register("base_css", Bundle("scss/base.scss", filters="pyscss", output="css/base.css"))
-
-
-# commands
-# --------
-
-
-def migrate_databases():
-    """Creates the necessary databases if they don't exist already; or migrates them."""
-
-    from .core import migrations as core_migrations
-
-    with open_db() as core_db:
-        try:
-            rows = core_db.fetchall("SELECT * FROM dbversions")
-        except sqlite3.OperationalError:
-            # migration 3 wasn't ran
-            core_migrations.m000_create_migrations_table(core_db)
-            rows = core_db.fetchall("SELECT * FROM dbversions")
-
-        current_versions = {row["db"]: row["version"] for row in rows}
-        matcher = re.compile(r"^m(\d\d\d)_")
-
-        def run_migration(db, migrations_module):
-            db_name = migrations_module.__name__.split(".")[-2]
-            for key, run_migration in migrations_module.__dict__.items():
-                match = match = matcher.match(key)
-                if match:
-                    version = int(match.group(1))
-                    if version > current_versions.get(db_name, 0):
-                        print(f"running migration {db_name}.{version}")
-                        run_migration(db)
-                        core_db.execute(
-                            "INSERT OR REPLACE INTO dbversions (db, version) VALUES (?, ?)", (db_name, version)
-                        )
-
-        run_migration(core_db, core_migrations)
-
-        for ext in valid_extensions:
-            try:
-                ext_migrations = importlib.import_module(f"lnbits.extensions.{ext.code}.migrations")
-                with open_ext_db(ext.code) as db:
-                    run_migration(db, ext_migrations)
-            except ImportError:
-                raise ImportError(f"Please make sure that the extension `{ext.code}` has a migrations file.")