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/commands.py

@@ -0,0 +1,51 @@
+import click
+import importlib
+import re
+import sqlite3
+
+from .core import migrations as core_migrations
+from .db import open_db, open_ext_db
+from .helpers import get_valid_extensions
+
+
+@click.command("migrate")
+def legacy_migrate():
+    migrate_databases()
+
+
+def migrate_databases():
+    """Creates the necessary databases if they don't exist already; or migrates them."""
+
+    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 get_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.")