PDF functionality is now read-only. Auto generating pdf's
function to be craeted.

Needed refactoring of submit manuscript flow.
Checks are run via the forms. Also, the copying
of data from old submissions is done via forms.

All this functionality is removed from the model.

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.

First working, not-tested, version of a multi-journal SciPost.
Will rebuild all views to CBVs to use mixing for the recurring
`journal` context variable.

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...

Prepare ArxivCaller to be used in multiple models.
 It includes the creation of a ArxivCallerMixin in the
 behavior file which should be used in all objects using
 the ArxivCaller.