rest: clean up ModelSerializer.get_default_fields

[sheppard]

The multiple interwoven use cases are getting a bit out of hand. Probably should break down and have multiple serializers for each case:

• Detail view (retrieve/update)
• List view (list)
• List view post (create)
• GeoJSON versions?

Perhaps the view should then have more specific functions like get_serializer_for_update?

Edit: The biggest issue is really the excessive magic for generating nested serializers. Instead of detail/list/etc. serializers, this can be cleaned up simply by creating explicit, custom serializers whenever a nested serializer is needed. This will likely affect the external API so is a good candidate for 0.8. See #34.

[sheppard]

This should probably be fixed in conjunction with #11

[sheppard]

Also see #32 which will likely involve similar changes.