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.

Handle ItemNotFoundError from the modulestore to avoid 500 errors

Fix/vik/oe cache

Sandbox-installed packages will be really installed instead of -e instal...

Fix advanced components list in Studio.

Use draft module store so we can test editing components (problems, html...

Add Review Board config to repo

Upgrade version of xblock.

Use a different arbitrary dotted module, which Christina doesn't already...

Fingerprint the filenames in site-packages to decide when to install req...

Fix/cale/pylint cleanup

Handle Heroku's 503 maintenance mode response

The LMS comment client previously would try to parse the response
as JSON, choke, and return a 500 to the client. Now, the LMS client
displays a message indicating that the forums are down for
maintenance.

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

Studio - adding back in older Sass lh() function/mixin

studio - adds back in older lh() Sass function/mixin and moves it to the inheritied/to-be-sunsetted mixins sheet

actually needs to be 255 given unicode requirements

A better way to compute the hash, that gets the right hash after an install actually installs something.

Bugfix/e0d/resize notes uri