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