In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19625 some
changes were introduced that do not meet our abstraction reuse rules.
This commit cleans up some of these changes so the requirements are met.

Most notably, sorting of the runners in Admin::RunnersFinder has been
delegated to Ci::Runner.order_by, similar to how we order data in
models that include the Sortable module. If we need more sort orders in
the future we can include Sortable and have Ci::Runner.order_by call
`super` to delegate to Sortable.order_by.