DOIs for Reports and Comments which relate to published
objects (at SciPost or elsewhere) to now make use of
Crossref's peer review content.

All required actions are determined and saved in `submission.cycle`
now. The actions required are determined at initialization of
the submission. Different cycles now possibly can have different required
actions. All template logic to determine required actions is removed
from the codebase.

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...

The Comment model now has a option to
 append a file (pdf, png, jpg) to itself.

Improve comments app to follow PEP rules
 and remove all wildcard imports!!