Include exemption for "current quarter" in determining if term is accessible

[psvenk]

We have some code to determine if a term is "accessible" on Aspen; that is, if it has any grades. However, at the start of a new term, the current quarter is deemed inaccessible because no teachers have entered any grades, having the effect that once a user switches to a quarter other than "current quarter", they are unable to come back to "current quarter" due to its being marked inaccessible. We should include an exemption for "current quarter" (and, once #82 is resolved, the quarter — Q1, Q2, Q3, or Q4 — that corresponds to the current quarter) to avoid this situation.