Wagtail-photo-gallery

Fixed `CollectionChooserBlock` by adding `get_form_state()` (Issue 2):

py
class CollectionChooserBlock(blocks.ChooserBlock):

def get_form_state(self, value):
return self.widget.get_value_data(value)

Image Gallery for displaying images with *CSS-flexbox*, managable via Wagtail admin.

Models:
`ImageGalleryMixin`: extention to display galleries within Wagtail pages,
`GalleryBlock`: Display all albums belonging to a certain collection,
`Album`: Grouping of images,
`AlbumImage`: Concrete Image with automatic resizing and thumbnail

Wagtail Admin:
- Preview cards for Inlinepanel
- `SelectAndSort` Drag and Drop sorting with selection mode