SQLAlchemy engine configuration is not passed to FAB based UI. Faced this issue when running Airflow with MySQL metadata store with wait_timeout = 120. Webserver is failing with Internal Server Error due to "[MySQL Server has gone away](https://docs.sqlalchemy.org/en/13/faq/connections.html#mysql-server-has-gone-away)" error. Settings like `sql_alchemy_pool_recycle` and  `sql_alchemy_pool_pre_ping` have no impact on FAB internal communication with MySQL.

**Apache Airflow version**: 1.10.12
airflow/settings.py
"""
    This policy setting allows altering tasks after they are loaded in
    the DagBag. It allows administrator to rewire some task parameters.
    To define policy, add a ``airflow_local_settings`` module
    to your PYTHONPATH that defines this ``policy`` function.
    Here are a few examples of how this can be useful:
    * You could enforce a specific queue (say the ``spark`` queue)
        for tasks using the ``SparkOperator`` to make sure that these
        tasks get wired to the right workers
    * You could enforce a task timeout policy, making sure that no tasks run
        for more than 48 hours
    * ...
    """
def task_instance_mutation_hook(task_instance):  
    """
    This setting allows altering task instances before they are queued by
    the Airflow scheduler.
    To define task_instance_mutation_hook, add a ``airflow_local_settings`` module
    to your PYTHONPATH that defines this ``task_instance_mutation_hook`` function.
    This could be used, for instance, to modify the task instance during retries.
    """
def pod_mutation_hook(pod):