CLC-3303-finaid-messages-older-than-one-year-appear

https://jira.ets.berkeley.edu/jira/browse/CLC-3303

Now making sure that no document messages with dates older than
one year are being shown.

Things I did:
- Stopped using the xml Aid Year altogether.  The proxy term_year
  already has that information.
- Stopped passing the cutoff_date around as a parameter since it's
  now returned as a singleton in a class method.
- Determine the cutoff_date in a method that bases the cuttoff date
  on the current time: (Time.zone.now - 1.year)
- Moved all methods except for append! to private and alphabetized them.
- Wrote a spec for the my_fin_aid model that checks that entries
  with dates older than one year are not added to the activities feed.