Move lms.lib.xblock into lms.djangoapps.lms_xblock in preparation add database backed configuration to it

Check settings.FEATURES["ADVANCED_SECURITY"] first

This commit pulls in changes from #4487 that implements keyword
substitution for bulk emails. With these changes, an instructor can
include keywords in their bulk emails which will be automatically substituted
with the corresponding value for the recepient of the email. Keywords are
of the form %%keyword%%, and the keywords implemented in this commit include:

%%USER_ID%% => anonymous_user_id
%%USER_FULLNAME%% => user profile name
%%COURSE_DISPLAY_NAME%% => display name of the course
%%COURSE_END_DATE%% => end date of the course

Client-side validations have also been implemented to ensure that only emails
with well-formed keywords can be sent.
The architecture is designed such that adding in new keywords in the future
would be relatively straight-forward.

We do this intentionally, to preserve tracebacks

TNL-884

This reads configuration directly from yaml
and imports it directly into settings

It might be different from the requested filename.

Tests in common are run with both CMS and LMS configuration (with different env files). Ensure the DEFAULT_FILE_STORAGE variables are in both environments.

This should always be overridden by the testcases if needed.

TNL-735

Fixed the edX middleware to lazily create request
context for Mako templates, so that it is based on
any changes made to the request object.

Verified that with this fix the "Instructor" tab is
correctly hidden when a staff member is viewing
the course as a student.

OPEN-328

OPEN-328

spinner.gif and spinner-on-grey.gif are the same file.
Explicitly give the LMS a spinner.gif file.

OPEN-328

OPEN-328