Add Wagtail hooks to allow various further elements through its whitelister
Add Wagtail hooks to allow various further elements through its whitelister

Fixes #61

terotic/digihel,City-of-Helsinki/digihel,City-of-Helsinki/digihel,City-of-Helsinki/digihel,terotic/digihel,City-of-Helsinki/digihel,terotic/digihel