Migrate from Google search to MkDocs.

This is optional, but will be required by MkDocs 1.0

first time

This change updates the modal to use MkDocs for search.

The {{ page_title }} and {{ site_name }} variables are now separated to
add flexibility th themes.

This allows us to delete lots of code and avoid hard-coding the
navigation. This does mean that {{ current_page }} isn't always
available and needs to be conditionally used.

Fix minor typo

Update validators.md

Missing colon.

Added test ensuring DecimalField accepts `2E+2` as 200 and validates decimal place correctly.

THe DRF Any Permission is not maintained anymore

Use shields.io instead of pypip.in

Set the action on a view when override_method regardless of its None-ness

Add drf-tracking to third-party-resources.md

Add drf-tracking to third-party-resources.md

Update mkdocs version to 0.12.2

Fix markdown link references in release notes

The following items had a wrong href value:

- Dont require pk strictly for related fields. (#2745, #2754)
- Restrict integer field to integers and strings. (#2835, #2836)

Add Classy Django REST Framework(cdrf.co) to Third Party Resources

The old version of mkdocs required 'Markdown>=2.3.1,<2.5' and
requirements-optionals.txt lists markdown==2.5.2 causing mkdocs
to fail.

Support basic authentication with custom user models that change username field

On Tom's suggestion, improve coding style by using a single-line call to getattr() with a default value instead of a multi-line if/else clause.

Fix flake8 error

Support User model in Django 1.4 that has not a USERNAME_FIELD attribute.

Import get_user_model from compat module to be compatible with older django versions (e.g. 1.4).

Compatibility code for getting user model

Support basic authentication with custom user models with a username field that is not named 'username'.

decimal place correctly.

Allow unexpected values for ChoiceField/MultipleChoiceField representations