Don't add disabled class to loading button

- Moves toggle button to a shared location
- Adds tests for toggle button
- Transforms Clusters class into function
- Improves UX

Merge branch 'list-multiple-clusters' of https://gitlab.com/gitlab-org/gitlab-ce into list-multiple-clusters

* 'list-multiple-clusters' of https://gitlab.com/gitlab-org/gitlab-ce:
  Fix symbolization error in #clusters
  Add specs for scoping in ClustersController
  Add pagination specs for ClustersController
  Add specs for cluster counters
  Use current_user in ClustersController#clusters
  Add specs for ClustersFinder

Finishes tabs & table
Updates e2e tests