Add endpoint to set abort flag for slots
The new endpoint is of the form
/ajax/abort/nightly/slots/lhcb-head.2470;lhcb-head.2471/
following the existing rebuild
endpoint.
When requested, the CouchDB document for each slot is updated
by setting the top level key "abort" to a dict containing the
time when the abort was triggered.
The presense of a truthy value under "abort" will be the signal to stop the checkout/build/tests.
Part of LbNightlyTools#18 (closed)