MediaGallery (#2371)
MediaGallery (#2371)

* Add new MediaGallery type

From proposal #2139 

Creating a new parent type, MediaGallery for the subtypes of ImageGallery and VideoGallery, to allow for a mixed-media collection page type.

* Subtypes for MediaGallery

Making IMageGallery and VideoGallery subtypes of MediaGallery

* Fixing mistaken entry from earlier commit

schemaorg/schemaorg,schemaorg/schemaorg,schemaorg/schemaorg,vholland/schemaorg,schemaorg/schemaorg,vholland/schemaorg,vholland/schemaorg,schemaorg/schemaorg,vholland/schemaorg