Tiebreaking on last enrollment numbers

[hsheth2]

There's a couple scenarios where we'll want to modify the last_enrollment field algorithms:

• When there's multiple sections with different professors (e.g. MATH 120, where Sudesh tends to have larger classes than other profs)
• When a previous professor comes back to teach a class that they previously taught, but didn't in the most recent instance (e.g. Stan with CPSC 323)

[Follow up on #37, cc @kevinhu]