A new filtered runningcatalog view (#5952)

This implements a new filtered running catalog view, described in this issue:

https://support.astron.nl/lofar_issuetracker/issues/5952

Key concepts:
 * It uses django-filtering to generate Django forms and filter results
 * For the more complex (impossible in django ORM) queries it uses a view in the database
 * Removes the transient ng view (assocxtrsource view)
 * small cleanup here and there.


this comment doesn't make sense, i'll remove it tomorrow.