CLC-3384 Annotating Financials feature code, and standardizing some techniques

https://jira.ets.berkeley.edu/jira/browse/CLC-3384

Annotate the Financials feature so it's easier to follow for new devs. Adopt the before_filter style of authenticating API endpoints throughout. Rewrite Financials specs to use RSpec 3 style ("expect" not "should"). 

These rewritten tests pass in RSpec 3, but I haven't switched the whole app to RSpec 3 yet, because there are test failures in other tests. Still, for all new code, we should use the Rspec 3 style.