LP#1815815: Simplify basic search UI

LP#1815815: Simplify basic search UI

This commit adds logic to the org selector (and backing context building
logic) to allow the library dropdown to render Library Groups in a way
mechanically similar to location groups.  This is used in the basic
search interface to simplify the Library Group feature.

The advanced search interface still uses separate Library and Where
dropdowns, as this allows advance search to retain the depth selection
capability, and more directly shows the (real) relationship between a
library and the location groups, library groups, and depths that are
relevant to it.

Signed-off-by: Mike Rylander <mrylander@gmail.com>