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.

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

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.

Potential NotDefined raise if type is neither a
 `test` nor `deposit`.