Semester property rework

[jameslzhu]

The Property.current_semester method returns the current semester, string encoded as YYYYS, with a four-digit year YYYY and one-digit semester S (1 for spring, 2 for summer, 3 for fall). (It's a getter method for the Property.semester attribute.)

It's tied into many major mechanisms:

• Elections: setting the semester attributes of new Committeeships
• Assistant officer / committee member additions: ditto
• Officer view pages: determining which semester to view
• Course reviews: separating them by semester
• Resume books: ditto
  ...

There are, however, a few things that we don't want tied to the current semester:

• Elections: we hold elections before the next semester
• Assistant officer / committee member: we want to be able to add them now or in the next semester

There are also a few things we might want to tie to the current semester:

• Dept tours: Allow dept tours to be closed for the summer #193

There is one major issue with all of this: the current semester is rolled over manually, with the Ruby console, ssh'd into the apphost.

We'd like to:
a) Tie the current semester to the actual current time
b) Decouple certain things from using just the current semester (elected positions)
c) Tie the department tour closed notice to the current semester

There may be more things to do here, but I'll add them later.