Officers API and data

[wilt00]

Implements #12. Note that this implementation differs from #11 in the following ways:

• I really don't like "rank", it feels too hierarchical for LUG. I've used "position" instead throughout. I can change this back if it'll be too much of a hassle to change everything else.
• Officer objects now have the additional members term_start and term_end, representing the start and end of their term of office. For a normal term of office, these run from Summer of one calendar year to Spring of the next, inclusive. These are stored in the form SEASON_YYYY, e.g. SPRING_2018.
• The route /api/officers provides the following query parameters:

  /api/officers?position=[PRESIDENT,VICEPRESIDENT,TREASURER]
                Officers holding the given position
             ?semester=SEASON_YEAR
                Officer was in office during the given semester
                e.g. "?semester=SUMMER_2018
                Overrides 'year'
             ?year=<Academic year>
                Officer was in office during the given calendar year
                Note that, as some officers do not serve a full calendar year, this may
                return multiple individuals for a position
                e.g. "?year=2018" for Summer 2018 - Spring 2019 inclusive
                Not read if 'semester' param exists
             ?is_current=[Y,N]
                Return only current students

  (Now I look at it, is_current is confusing, people might think it means "is currently serving" rather than "is currently a student".)

Coming out of this, I'd like to get proper logging and testing set up, those would've been helpful when developing this. I'll make issues for those.

[wilt00]

Changes made. I'm pushing the is_current_student query param to another PR, since that might need some extra logic around term dates.

[wilt00]

We'll definitely split Officer out, but I think the way we want to do that is to give each API category (officers, events, etc) its own models/views/controllers. That's a big enough change that I think it deserves its own PR.

[BluuArc]

Alright. We can create another issue surrounding the file structure of the backend (in addition to the is_current_student parameter) after this PR is merged.