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

* master: (22 commits)
  static-analysis fix
  refactored the hashed repository name
  removed 'only once' from the docs
  static analysis fix
  added changelog
  ignore hashed repositories when doing rake gitlab:cleanup:dirs
  Update prometheus Gem to fix compilation on centos 6 duo to GCC not recognizing `std=c11` option
  Add a feedback mechanism to `package-qa` manual action
  Update svg external dependency
  Drastically improve project search performance by no longer searching namespace name
  Deduplicate protected ref human_access_levels
  Fix ProtectedBranch access level validations
  Address feedback for AWS Runner autoscale article
  Clarify usage of Runner limit and AWS tags
  Finish rest of the sections
  Finish all sections except docker.machine
  Add runners.cache section
  Add Runner registration info
  Clear up sections
  Add skeleton
  ...

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:
  Simplify Clusters::CreateService cluster check

Upgrade jQuery to 2.2.4

Closes #40481

See merge request gitlab-org/gitlab-ce!15570

removed 'only once' from the docs

Closes #39261

See merge request gitlab-org/gitlab-ce!15601

ignore hashed repositories when doing rake gitlab:cleanup:dirs

Closes #40352

See merge request gitlab-org/gitlab-ce!15600

Update svg external dependency

See merge request gitlab-org/gitlab-ce!15596

Add tests
Update empty state
[ci skip]

Add article on autoscaling GitLab Runner on AWS

Closes #39650

See merge request gitlab-org/gitlab-ce!15313