Move base enumerations to base model and metadata page refactoring, refs #892

* Added a METADATA_DOWNLOAD_ALLOWS settings
* Moved CONSTRAINT_OPTIONS to database, added initial fixtures and migrations
* Moved SPATIAL_REPRESENTATION_TYPES to database, added initial fixtures and migrations
* Added gn_description and is_choice to TOPIC_CATEGORIES, added new fixtures and migrations
* Moved COUNTRIES (keyword_region) to database as 1-n, added initial fixtures and migrations
* In the Fields metadata section hided the Display Title, Display Order and Visible attributes (so far they are not used withing GeoNode). Added the field Description to the layer_attribute model, that can be used for metadata purposes, and updated migrations