Implement search box functionality

This PR implements the search form, so that it works as expected.

* Searching for a keyword, e.g. "postal", will search agencies' **name**, **slug**, **abbreviation**, and **description**. If there are 2 or more results, it will filter the list of agencies, as seen in the screenshot below. The search box will be pre-filled with the search term.
* If there is 1 result, it will redirect the user immediately to the resulting agency's contact landing page.
* If there are 0 results, it will bring the user to `/agencies/` and will say "There are no agencies matching your search." in place of search results.

I've added tests for the above 3 cases.

I've also ripped out [`typeahead`](https://twitter.github.io/typeahead.js/), the autocomplete plugin we were using, after discussion with the design team. There is no autocomplete functionality now.

Fixes #294, #242.