Add a dependency for finaid migration

It needs the accounting models to be set up.
Fix memory leak in accounting migration for some setups

On my 32-bit Python 2.7 install it was running out of memory.  I found
that this was caused by Django caching database queries, and could be
fixed by flushing that cache.  I also moved the migration for individual
programs into a separate function.

Reminder: this migration takes a long time, and existing accounting data
may be very messed up.