Use constant and helper to load erb search_form.

This PR addresses the issue outlined in https://github.com/resque/resque-scheduler/pull/378. The fix there in fact worked for me as well, and this patch is based on that solution as well as https://github.com/quirkey/resque-status/commit/fc3fe3680c98569d444d60201dcd945d23d292fb.

This branch is based off the v2.5.5 tag -- I know resque-scheduler's at 3.0.0, but if this could get backported/released as v2.5.6, that would awesome.

cheers