Force notifications to SAML email address for dedicated users

What does this MR do?

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/5332

This MR needs to wait for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9663/diffs

CE backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25739

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

app/views/profiles/notifications/_email_settings.html.haml

-= form.select :notification_email, @user.all_emails, { include_blank: false }, class: "select2"
+- form = local_assigns.fetch(:form)
+.form-group
+  = form.label :notification_email, class: "label-bold"
+  = form.select :notification_email, @user.all_emails, { include_blank: false }, class: "select2", disabled: local_assigns.fetch(:group_managed_account, nil)
+  .help-block
+    = local_assigns.fetch(:help_text, nil)