Fix for issue #890

Modified the potential_mentor_count to return the number of mentors in
the project. Also, changed the test and made the result.html to work as
paulproteus has suggested. Should the function be renamed to
get_mentor_count ?
I think if you're going to return a `count()` it's just as speedy if you don't jump through the `values_list()` hoops, and so you could change that if you want to make the code cleaner.

Almost surely we should be finding out what exact SQL query Django is sending to the database for this, to confirm or deny my suggestion.