Django-clubhouse

- GalleryBlock inheritance fix.
- Fixed Zipfile upload on GalleryBlock

- Added serializers for contrib pages
- Added serializers for contrib blocks
- Added serializers for contrib block contexts
- Fixed contrib.ModularPage.content property
- Fixed contrib.ModularPage.aside property
- Fixed contrib.GroupingBlock.content property
- Added `override_default` Meta option on BlockSerializer

- Autodiscovering serializers modules when clubhouse.core.serializers is loaded

- Minor fix on grouping block template
- Added BlockSerializer for djangrestframework which adds default_serializer
and serializers list to block models
- Added BlockContextSerializer which adds the block_object to serialized data

- Added render_blocks template tag to clb_tags
- Updated all block templates to fit with the render_blocks tag
- added help text to BlockContext.additional_classes
- BlockContext has get_template for the context template, defaults to
model_name.html
- BaseBlock has get_template for the block template, defaults to
blocks/model_name.html

- Added AbstractModularPage which implements a method to get blocks by context
model