Enable alert_query transmission to the backend

- Added `alert_query` to be sent in the create / update request
  to the backend

- Added alert_query to prometheus_alerts table - allow nulls

- alerts can be added and updated

- added both (!ID) for specifying custom metrics to be alerted on

- PrometheusAlert expands (!ID) to the appropriate prometheus query
  ... added tests for that

- AlertParams converts (legend) into (!ID) so that the abbreviated
  query and full query can be distinquished and sent to the correct
  place

- abbreviated_query has replaces the (!ID) with the legend instead of
  the query