Added BoxIndex for searching boxes.

Branch: Added a BoxIndex multimap index to see how it would work for
searching boxes compared to some of the other options I have tried. The
index is working and returning results, but more work needs to be done
in SearchForm to get this fully functional. The index might also need to
be tweaked, perhaps adding a TransformResults to get back full Box
instances.
BoxIndex-based searching fully implemented.

- Added TransformResults to BoxIndex to get back BoxSearchResult
instances
- Updated TreeNodeCollectionExtensions to support BoxSearchResult and
restored DisplayName usage
- Updated SearchForm to work with BoxSearchResult and to use a separate
session for each search query