HEAD requests not supported

[sentry]

Sentry Issue: ZEUS-G8

KeyError: 'HEAD'
  File "zeus/api/resources/base_revision.py", line 36, in dispatch_request
    revision.repository_id, PERMISSION_MAP[request.method]
...
(6 additional frame(s) were not displayed)

KeyError: 'HEAD'

[dcramer]

Not entirely sure what we should do here, but we could just require GET permissions.

Same concern around OPTIONS.