PLA-275 -- refactor video embed tool ( and search API controller )

This updates the VideoEmbedTool extension to use a service class to handle searches, rather than a controller. This allows us to implement unit tests to provide additional guarantees on the functionality of code around not only the search component but other VET components as well.

Also included are changes the search API controller, to make it more testable, which are required for the updates here. These changes were originally part of https://github.com/Wikia/app/pull/810, but I have compacted them into a single pull request for convenience, and due to interdependence of code.