Add new combined search box setting: collapseInactiveBackendOptions

[demiankatz]

This feature was requested at Villanova to make the search drop-down menu more manageable -- rather than showing every option under every backend, we can configure to show only sub-options for the active backend. This makes search a little less flexible (it takes multiple clicks to get to a narrow search in a different backend) but it makes the overall experience less intimidating.

To clarify, here's an example search box with default combined handler behavior:

And here's the same search box with the new setting turned on:

TODO

• Add test coverage