add some basic inheritance to search adaptors

provides support for future subclasses (alphabetical search adaptor)
extract adaptor initialization to concern

allows for shared adaptor initialization in multiple controllers

adopt some rails 4 defaults regarding folder structure for controller
concerns
(https://github.com/rails/rails/commit/f6bbc3f582bfc16da3acc152c702b0410
2fcab81), need to require this manually as we're still on rails 3