Add Reversion and management links

This MR adds the functionalities of django-reversion to the website. Every edit (that does not come through the admin-interface) now sends a link to the database managers with a link to view all the changes. Furthermore, I implemented several new views:

History page

We have a new history/ tab where all the revisions can be accessed

Revision detail

Each item in this list links to the detail view, where the individual models that have been changed are listed

Revision admin

Clicking on an arrow leads to the revision on the admin page.

If you'd want to revert this, you have to select an earlier revision from the history. It would be better to revert it on the site, but that's not yet possible so far.

In this history, you also see the various versions

Object specific revision list

Another feature is the object specific revision list. When you go to topics/HZG-001/history for instance, you'll only see the revisions that touched this object.

Staff users can also access these history pages through the little clock button on the upper right side

closes #23 (closed), #24 (closed)