Allow Airflow DB upgrades/migrations to be on-demand

[adwk67]

Related to SUP-174

At the moment, upgrades to the airflow database occur every time the scheduler starts up: see here and here. This can cause significant overhead, depending on the number and complexity of DAGs (as well as delays in the scheduler being available). This ticket will cover the following:

• replacing airflow db upgrade with airflow db migrate (as the former has been deprecated)
• making the migration on demand via e.g. a new field in the resource