Allow playlist tracks to be saved to the verification table at the time of playlist writes.

This is turned off by default due to the time cost imposed, but it may be useful in some circumstances.

As part of enabling this operation, the functionality to save search results is publicly exposed in the search module.
Update client-side Underscore to 1.4.4.
Update Backbone to 1.0.0 and adjust Muxamp usage to keep existing behavior the same.