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.

CAUTION! READ ME!
This migration will setup a proper attachment environment.
This means, that for existing files (journal publication!),
 these folders/files should be completely moved to the MEDIA_ROOT
 folder, else this will return a 404 NotFound for the related
 files.

Split scipost app into commentaries, comments, contributors, journals, ratings, reports, scipost and submissions. Cleaned up all models, urls and views.