retract/unretract comment can now respond with 404 when bad entry

It will respond with a 404 if the entry identified in the URL doesn't exist, isn't a UUID or isn't accessible by the authenticated user.
retract/unretract comment can now respond with 404 when bad comment

It will respond with a 404 if the comment identified in the URL doesn't exist or isn't a UUID.
retract/unretract comment can now respond with 400 when retracted param bad
retract/unretract comment can now respond with 409 when multi params

When the retracted query parameter is specified multiple times with different values, a 409 is returned.
metadata queries explicitly conv vals to strings.
refactored retract/readmit comment 403 checks
Merge branch 'dev' of https://github.com/iPlantCollaborativeOpenSource/DiscoveryEnvironmentBackend into dev
fixed retract/readmit comment bug

If the comment and entry both exist with the correct authenticated user permissions, if the comment isn't attached to the entry, a 404 is returned.