Added some fixes in the journalization of document modification:

 - Fix that separate metadata changes are journalized as metadata changes now instead as `file and metadata` changes .
 - Always create a separate journal entry for public_trial changes
 - When changing only the `public_trial`, don't create an entry for metadata changes. (fixes #449)
 - Reworked tests for the document modification journalization. :sweat_smile: 

@lukasgraf please have a look ...
This reads a bit weird. Basically the assertion right now is "Last journal entry after editing document without changing metadata is `"Document added"`". This is currently correct, but that's not really the point of this test, is it? The point is that there's no *new* journal entry, whatever the last one was. So if we change the journal entry on adding a document to "Document created" this test will fail. 

So I would probably check the length of the journal before editing, and after editing it should still be the same (no new event).