Working venue filter for matko locations.
Move common code to importer baseclass.
Helmet importer: a start.
Kulke: preliminary import.
Small fixes.
Make HelMet importer get or create DataSource in Importer.setup()

This way we don't rely on pre-existing fixtures, but rather create the
necessary data as soon as the importer is run the first time.
Kulke: Implement place matching to tprek
Merge branch 'master' of https://github.com/City-of-Helsinki/linkedevents
Fix context related problem with location field