Extend PostPublicationController to support PDF extraction

[Robert Jäschke]

This is a subtask of issue #2836.

The current PDF extraction code needs to be replaced by code that uses Grobid to extract the metadata and enrich it with data from BibSonomy's database (see other tasks of this milestone).

• A list of posts shall be extracted an then the handling should continue as it is now.
• An important task will be error handling: how can Grobid's parse errors (e.g., missing fields, etc.) be communicated to the user and user input be handled?
• In the first version, we probably have to ignore posts with errors (e.g., users can not edit them but will see the posts with their error messages and the posts will be disabled for storage).
• Users should always be required to edit the posts before they are stored (disable support for direct storage in this case! - ensure that command.isEditBeforeImport is true).
• The controller should use the batch edit view for this task.
• There's a lot going on in the method completePosts and other code after parsing. Check that this works correctly.