Use .get instead of [] to access env hash

Capture the exit code of the Mako template engine invocation on
asset preprocessing and abort from the Rake task on failure. This
will prevent the LMS from continuing its attempt to start up,
preventing further configuration errors.

If the `THEME_NAME` environment token isn't defined, then the
application.scss.mako file will crash during preprocessing. Use
.get to avoid a crash.

add MITX Feature setting to enable django admin site

Check functions need access to course-local code also, so we need to sav...

Add theming settings and simple base Sass preprocessing.

Per feedback from @cpennington. This way we'll only preprocess once,
even if the Sass compilation is invoked multiple times.

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.

Reviewed by cpennington: https://rbcommons.com/s/edx/r/2/

According to someone from Datadog, this was generating tags like "knowledgeable_
people_who_put_this_course_together._this_is_harvard._you_can_t_tell_us_there_s_
a_shortage_of_editorial_talent." They say that they can handle tens or hundreds
of unique tags but not thousands. Given that we have a unique URL for each
thread, we can't even use that as a tag. Thus, all tags are removed for now
until we can determine whether there is a useful set of tags with small enough
cardinality. In light of this, I did not investigate why the long tag mentioned
above was being generated.

Handle ItemNotFoundError from the modulestore to avoid 500 errors

Check functions need access to course-local code also, so we need to save the python path from the problem setup, and use it for the check function.

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