Implements keyword sub feature for bulk emails

This commit pulls in changes from #4487 that implement 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 recipient 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.
This should get wrapped in an if that checks whether the map has already been updated to non-empty values. Or perhaps such a check should be moved inside ```add_keyword_function_map()```. And once the map has been filled out, the update functions should be monkey patched to no-ops, like lambda x: x or something.

One of the themes of monkey patching is that you're doing evil, so you want to do it exactly once, and probiti further evil.