Allow search_builder_class to be passed to SearchService

[jcoyne]

So that this can be set without having to override SearchService. Since the SearchService is a new class in 7.0, this can be merged without concern for backward compatibility.

[coveralls]

Coverage decreased (-0.002%) to 94.742% when pulling b3d5304 on search-builder into 5284e21 on master.

[coveralls]

Coverage decreased (-0.002%) to 94.742% when pulling b3d5304 on search-builder into 5284e21 on master.

[coveralls]

Coverage decreased (-0.002%) to 94.742% when pulling b3d5304 on search-builder into 5284e21 on master.

[coveralls]

Coverage remained the same at 94.743% when pulling bf728db on search-builder into 5284e21 on master.