Update Titel model to use TitelBevat directly

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.
Merge branch 'develop' of github.com:jvdzwaan/EmoVis into develop