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 ?
Will this print "0 mentors, and 1200 Python mentors" for a lot of projects? If so, that's a bit disheartening, I feel. Maybe if the potential_mentor_count is 0, then we should skip that phrase.

Also, we could rename from `.potential_mentor_count` to `.mentor_count` since it's now specifically about project mentors.