Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/40825

Closes #37595

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/40543
Bug introduced in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15192

* new merge request can be created by sending an email to the specific
email address (similar to creating issues by email)
* for the first iteration, source branch must be specified in the mail
subject, other merge request parameters can not be set yet
* user should enable "Receive notifications about your own activity" in
user settings to receive a notification about created merge request

Part of #32878

See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3566

Adds e2e tests

Don't add disabled class to loading button

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/38962

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

Finishes tabs & table
Updates e2e tests

Add tests
Update empty state
[ci skip]

added getter specs

fixed scss-lint