bug 1063150 - Add specification maturity to API
bug 1063144 - Add specifications to API
bug 1063144 - Partial PEP257 docstring compliance

Triple single quotes (''') to double quotes (""")
Single-line docstrings pulled into a single line.
Other small changes.  The pep257 checker still finds many, many issues,
but those are currently ignored in setup.cfg.
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

TranslatedField due to differences in section number across languages
bug 1063144 - Add sample data
fix bug 1063144 - Add specs to browser, refactor

Refactor to use controller methods instead of template logic