Added Flask-Demo and Flask AppBuilder tutorial

fab/app.py

@@ -0,0 +1,17 @@
+"""Initialize the Flask application, database session, and appbuilder object."""
+
+from flask import Flask
+from flask_appbuilder import AppBuilder
+from flask_appbuilder.utils.legacy import get_sqla_class
+
+app = Flask(__name__)
+app.config.from_object("config")
+
+db = get_sqla_class()()
+appbuilder = AppBuilder()
+
+with app.app_context():
+    db.init_app(app)
+    appbuilder.init_app(app, db.session)  # type: ignore
+
+    import views  # noqa