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 ...
Should be `PUBLIC_TRIAL_MODIFIED_ACTION`, no?