Extend batch edit view for Grobid PDF extraction

[Robert Jäschke]

This is a subtask of issue #2836.

In the first version, it should be possible to just use the batch edit view as it is now.

Later on, we probably want to support the user by

1. highlighting the different types of posts (one post represents the PDF itself, all other posts represent its references (citations)). The first post should always be the one representing the PDF itself.
2. disabling posts that have errors or are incomplete.
3. allowing users to complete/fix broken posts (we would need a completely new, dynamic view for that).