Adds `#build_notification_recipients` to `NotificationRecipientService`
that returns the `NotificationRecipient` objects in order to be able to
access the new attribute `reason`.

This new attribute is used in the different notifier methods in order to
add the reason as a header: `X-GitLab-NotificationReason`.

Only the reason with the most priority gets sent.

Calling `finder.execute.count` twice will perform two queries, but these are all
in the same file, so it's easy to just reuse them.

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

Closes #40824

Add GKE, Login - HTML & CSS
Cluster details - HTML & CSS

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/40671

See
https://gitlab.com/gitlab-org/gitlab-ce/blob/f7254a4060b30e3134c6cf932eaba0fc8e249e9a/app/controllers/sessions_controller.rb#L42
for an example of where we set `flash[:notice] = nil`

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

Finishes tabs & table
Updates e2e tests

In the new navigation header, when a user uploads a custom header
logo GitLab will still show the 'GitLab' type logo. This is often
undesirable as the user may want to upload a logo with their own
name present. This change will toggle the GitLab type logo if a
custom header logo is present.

Closes #39708