PDF upload dialoge for PDF extraction

[Robert Jäschke]

This is a subtask of issue #2836.

On the postPublication page I can already upload a file. The dialogue needs to improved and extended such that it can replace the view on https://www.bibsonomy.org/import/publications:

1. Add a nice dropdown area that allows users to use drag and drop to upload their PDF file (an example can be found here).
2. Migrate the form fields from /import/publications to postPublication.
3. Adopt the text to explain that BibTeX, EndNote, and PDF files are supported (it would be best to show the icons of the support types or something else that makes this very clear).
4. Allow users to choose whether they want to upload only the PDF file and extracts its author + title or whether they (in addition) want to extract and post the bibliographic references.
5. Allow the user to choose the visibility of the created posts (it's an effort to do this after posting).

This will also require changes in the controller and URL path configuration, as the /import/publications could be omitted.