Implement "filter by" and "limit to" search fields in the sidebar

[hawkrives]

"filter by" lets you filter down to just a specific group in the list, so if you group by term, you can only see that term; if you group by department, you can only see that department; etc. It's live with respect to your current "group by" option.

"limit to {year}" lets you easily limit the results to a specific year. It's applied first, so if you limit to 2012, you'll only see grouping options for 2012, etc.

---

Known issues:

• If you pick a filter and then change the year limit to a year without that filter key, you will get no results. (this no longer sounds like an issue. testing needed.) not actually the problem; see below
• if you filter by something, then change the grouping, it still tries to filter by the old key, which no longer applies to the new groups. (fixed by resetting filterBy when we change groupBy)