Create feed flush operation

[jamespfennell]

A feed flush operation would remove all updatable entities that have been added from a feed. It easily be implemented as a pseudo feed update where the feed parser is overridden to return an empty list.

The main use case is manually clearing out the stale data from feeds that have become unresponsive.

This operation should be executable via:

POST ./<system>/feeds/<feed_id>/flush

Additionally:

• Follow up ticket to add a new periodic task that automatically runs the flush operation after failed feed updates if there has been no successful feed update for X minutes. Configurable via the system config.

• Can probably be used to speed-up the system delete and/or update operation.

[jamespfennell]

Make sure to document the new endpoint

[jamespfennell]

....and make sure there's a sync vs async option

[jamespfennell]

...and write a new end to end test in test_updatestops or somewhere similar

[jamespfennell]

Done