This commit adds the requisite settings and startup features to
enable integration of themes into the edX platform. It does not
yet provide hooks in any of the templates, but it does cause the
main `lms/static/sass/application.scss` file to `@import` a theme's
base Sass. Template hooks will come down the road.

CHANGELOG
---------
Define a new `MITX_FEATURE`, `USE_CUSTOM_THEME`, that when enabled,
can be used in templates to determine whether or not custom theme
templates should be used instead of the defaults.

Also define a new setting, `THEME_NAME`, which will be used to
locate theme-specific files. Establish the convention that themes
will be stored outside of the `REPO_ROOT`, inside the `ENV_ROOT`,
in a directory named `themes/`. `themes/<THEME_NAME>` will store
the files for a particular theme.

Provide a function, `enable_theme`, that modifies the template and
static asset load paths appropriately to include the theme's files.

Move the main LMS Sass file to a Mako template that conditionally
`@import`s the theme's base Sass file when a theme is enabled.

Add logic to the assets Rakefile to properly preprocess any Sass/
Mako templates before compiling them.

Start linting envs files, add task to only pylint errors, and move pylintrc file to the correct location

This functionality requires the Zendesk URL, user, and API key to be specified
in django.conf.settings. Also, add a flag to MITX_FEATURES (enabled by default)
to control the endpoint and the front-end feature (yet to be added).

overrides what is set in env.lms.common using the lms.env.json
file

This reverts commit 793354a0.

Change AUTH_TOKENS index reference to a get so it doesn't explode the whole app if I did Puppet incorrectly.