adding exact phrase checkbox to search form

These changes add a parameter that allows users to search for exact phrases in the filestore, which is especially desirable when searching for things like page titles that appear in other pages.

I also added a checkbox in the sitenav template to make the feature more visible, but can remove that checkbox if desired, and leave the URL parameter in place.

You can test the feature on [this wiki](http://hist118.wcaleb.rice.edu). It may be worth noting that this will probably require additional changes to the `search.js` highlighting script in order for its behavior to work as expected for exact phrase searches versus keyword searches.