Add applications section to GKE clusters page

See merge request gitlab-org/gitlab-ce!15064

Conflicts:
	app/assets/javascripts/clusters/clusters_bundle.js
	app/assets/javascripts/clusters/components/application_row.vue
	app/assets/javascripts/clusters/services/clusters_service.js

CE repository mirroring changes backport

See merge request gitlab-org/gitlab-ce!15184

Added discussion_line_code value to note response and use it to query the right…

Closes #38677

See merge request gitlab-org/gitlab-ce!14981

Merge remote-tracking branch 'origin/refactor-clusters' into 36629-35958-add-cluster-application-section

Merge remote-tracking branch 'origin/38464-k8s-apps' into 36629-35958-add-cluster-application-section

Merge branch 'fix/sm/37299-environments-validate-uniqueness-on-url-and-can-cause-silent-deployment-failures' into 'master'

Remove unique validation from external_url in Environment

Closes #37299

See merge request gitlab-org/gitlab-ce!15202

Fix cron_parser_spec.rb (`master` branch failure on Nov 5th)

Closes #39817

See merge request gitlab-org/gitlab-ce!15196