fix bug 1080650 - Add support.protected
fix bug 1082006 - One support per version/feature

Uses unique_together Django model.Meta feature, which includes an index
on supported databases.

Also required fixing JSON API renderer to handle the non-field-errors
raised by a collision.  Pull request at:
https://github.com/kevin-brown/drf-json-api/pull/15
fix bug 1082023 - Update Support cache on save
fix bug 1082044 - Drop browser.icon

Also drop the support code for validating icon URLs