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
bug 1063150 - Add specification maturity to API
bug 1063144 - Add specifications to API
bug 1063148 - Add specification sections
bug 1063148 - Add sections to feature

A lot of gymnastics to avoid addining a M2M instance to the API:
- feature.sections is an ordered PK list
- historicalfeature.sections is a JSONfield list
- A m2mchanged signal is used to keep the cache up-to-date
bug 1063148 - Add sections.features links
bug 1063150 - Add maturity.specifications
bug 1063144 - Add specifications.sections
bug 1063144 - Split specifications.key

slug - traditional slug
mdn_key - the lookup key in
  https://developer.mozilla.org/en-US/docs/Template:SpecName
fix bug 1063150 - maturity.key to maturity.slug
fix bug 1063148 - Add optional sections.number
bug 1063144 - Add sample data
fix bug 1063144 - Add specs to browser, refactor

Refactor to use controller methods instead of template logic