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.
Just to throw in a note here, it might be nice to note the new functionality in the docstring as well.  And also to make more clear that the keyword substitution only takes place in the message body provided by the sender, and does not take place on the template itself.