Add ability to cancel observation requests

[griffin-h]

Is your feature request related to a problem? Please describe.
There is currently no way to cancel an observation request through the TOM Toolkit observation infrastructure.

Describe the solution you'd like
One suggestion is to add a "Cancel" button to the observation request detail view (URL /observations/<pk>/). This might only appear if the status is still PENDING. Clicking this button should send a request to the observatory to cancel the observation request and, if successful, set ObservationRequest.status='CANCELED'.

Additional context
Possibly related to #746 requesting observation deletion functionality, but different in the sense that this does not delete the record or any existing data.