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

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

Improve HTML structure for journals
 such that all elements are aligned with
 eachother and most content is repsonsive.
 Biggest thing to do still is the journal-menu
 which is not reposnive yet.

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`.

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.