Make DropdownArtist use a partial text string search

Depends on https://jira.mongodb.org/browse/SERVER-15090

## Description of Issue

Right now, you have to search "Beatles" in `DropdownArtist` to get "The Beatles" - you can't just search "Beat".

## Possible Solution and/or Approach

Once Mongo implements partial searches (https://jira.mongodb.org/browse/SERVER-15090), implement this in the Library search API. The Library search API should default to not doing a partial search, but should take a parameter that changes it to use a partial search.

Alternatively, this could be implemented on our own using a traditional query instead of a text query, but this is low enough priority right now that it can wait until Mongo implements this functionality.