Implemented Keyword Substitution feature

With this commit, instructors can include keywords in their bulk emails and
announcments that will be automatically substituted to the corresponding value
in the database. This commit introduces: %%USER_ID%% => anonymous_user_id and
%%USER_NAMR%% => user profile name.

This substitution in done by common/djangoapps/util/keyword_substitution.py. It
has a map between keywords and functions that will be called on the original
string (html document), and replace each keyword with the return value of the
function. In order to add more keyword substitutions, we can just add
"%%keywords%%" and their function mappings to the map in the future.

In the future, we can use this module to introduce keyword substitution feature
in other places.