Restructured app

[minimalparts]

This PR includes a lot of restructuring, meant to simplify the PeARS index. IDs linking database entries with numpy matrix rows are now included under the 'vector' attribute of the Urls model.

The PR also includes more minor changes:

• some styling of the admin database;
• direct indexing of URLs, without writing the information submitted by the user to a file first. NB: this very nearly means we could get rid of the userdata directory, but it is still needed to index pdfs, which are first (temporarily) downloaded and processed page-by-page;
• improved admin access to database entries: the admin can now make changes to a number of fields in the database entry, automatically triggering appropriate changes in the pod structure and matrix shapes.

[nandajavarma]

Thanks a lot for adding these cahnges @minimalparts ! Shall we merge this change?

[minimalparts]

Thanks for reviewing, @nandajavarma ! Yes, I'll merge now.