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

The mailing lists member sync was too elementary to
be used on production properly. This was for example
due to the single calls to MailChimp for each seperate
Contributor.

Now during sync, only updates are called to MailChimp, not
*all* contributors. Also, the batch operation is used instead
of seperate calls. Further, some query tweaks were used that
expect database calls to improve significant.

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.

Filtering is done by new template filter. Checking the position
of the current item in the forloop.

Migrations involve appending default values to model fields