Start transforming the prefill with Arxiv commentary request
 to a class based view using the Arxiv caller.

RequestCommentary is transformed to a class based view. Further
 moved strings to `/strings` and used django.messages middleware
 to pass messages to the user instead of context data.

Migrations involve appending default values to model fields

Move few constants to a seperate file away from models.
This implies minor edits in other apps relying on
these few constants.

Implemented basic modelmanager on Commentary
model. Created `vetted` and `awaiting_vetting` methods
and ipmlemented to few view methods.

Commentary model inherits from TimeStampedModel now. It
also includes migrations related to the inheritance:
 - Add added field on Commentary
 - Alter field last_activity on Commentary