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...

Front page will now show the last four publications of
 the current issue of SciPost Physics. Also reconstructed
 the .panels to .cards.

In the journals app, the Issue model is extended
 with a status field. This is published by default,
 but has a draft options such that a journal can be
 created without being shown. Incomplete issues
 can cause errors for visitors, so use the draft status
 if an issue is not "visitor-ready" yet.

The string method is not returning the right
 fields.

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