Rewrite AssignMentor template to build a context containing list of mentors.
Rewrite AssignMentor template to build a context containing list of mentors.

Due to the recent changes of deciding to use a list of mentors for a project
instead of separate single mentor property and a list of additional_mentors
property, we make changes to this template class to build a list of mentors,
their keys and names as the context and pass on to the HTML template that
renders the final select widgets.

rhyolight/nupic.son,rhyolight/nupic.son,rhyolight/nupic.son