Add foreign keys to TitelBevat model

The TitelBevat model contains a mapping from titles to the titles it
contains. Using this mapping, we can find the first year of publication
of a title, instead of some later year (when the title was republished
later). We want to use this first year of publication in visualizations,
etc. In order to be able to use the TitelBevat table/model for these
purposes, the foreign key relations to the Titel table/model were
updated. This step also involved reloading of the data in the table,
because there were links to non-existing titles. These have been
removed.
Add url mapping and view to display title metadata

Added an url mapping and a view to display a list of titles in the
corpus and the titles they contain. The contained titles are the actual
corpus we are interested in. The next step is to add a view to display
the results.
Add template to display titles contained in the corpus

A template was added that displays the titles in the corpus and the
titles contained in them, revealing the year of first publication. This
year is what we want to visualize, e.g., in the genre/subgenre
histograms.
Update

The Titel model was updated to have TitelBevat as a many to many
relation. This has the benefit that titles contained in a title can be
found directly through the current title, instead of though a TitelBevat
object. The corresponding view and template were updated to use this new
direct relationship.