Add an `airflow` route to the Meltano UI
Implementation of #557 (closed)
Depending on how Airflow web interface is setup, this might be done either by:
No-go because both Flask app live in different virtualenv, so crossing that isolation would remove the benefit of the isolation.
- proxying the requests
No-go because Airflow doens't expose static_url_path and/or builds wrong URLs when fetching its assets (/static).
We'd need to send a pull-request to Airflow to add proper support for that.
- iframing Airflow
Go: there is already a PoC for that, but it is now ~Blocked by #591 (closed)