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

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

 - group_managed_account = @user.group_managed_account?
 - help_text = group_managed_account ? s_("Your account uses dedicated credentials for the \"%{group_name}\" group and can only be updated through SSO.").html_safe % { group_name: @user.managing_group.name } : nil
-= form.select :notification_email, @user.all_emails, { include_blank: false }, class: "select2", disabled: group_managed_account
-.help-block
-  = help_text
+= render_ce 'profiles/notifications/email_settings', group_managed_account: group_managed_account, help_text: help_text, form: form