This commit contains the removal of html from 'the important' models
 or at least contains replacements for model methods such as `print_as...()`,
 etc. etc. Except for `header_as_li`, methods are not completely deleted yet.

> Fixed/Improved the nested comments and comments in reports. They were
 not shown before.

> Improved DRY principle in several templates/codes.

> Created Managers in the `submissions` app to filter i.e. "pool" submissions
 (so exclude potential author from judging on his own publication) for the
 pool view in the query; not in the template!

> More...

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