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/_email_settings.html.haml

+- form = local_assigns.fetch(:form)
+- readonly = @user.read_only_attribute?(:email)
+- email_change_disabled = local_assigns.fetch(:email_change_disabled, nil)
+- read_only_help_text = readonly ? s_("Profiles|Your email address was automatically set based on your %{provider_label} account") % { provider_label: attribute_provider_label(:email) } : user_email_help_text(@user)
+- help_text = email_change_disabled ? s_("Your account uses dedicated credentials for the \"%{group_name}\" group and can only be updated through SSO.") % { group_name: @user.managing_group.name } : read_only_help_text
+
+= form.text_field :email, required: true, class: 'input-lg', value: (@user.email unless @user.temp_oauth_email?), help: help_text.html_safe, readonly: readonly || email_change_disabled
+= form.select :public_email, options_for_select(@user.all_emails, selected: @user.public_email),
+  { help: s_("Profiles|This email will be displayed on your public profile"), include_blank: s_("Profiles|Do not show on profile") },
+  control_class: 'select2 input-lg', disabled: email_change_disabled
+- commit_email_link_url = help_page_path('user/profile/index', anchor: 'commit-email', target: '_blank')
+- commit_email_link_start = '<a href="%{url}">'.html_safe % { url: commit_email_link_url }
+- commit_email_docs_link = s_('Profiles|This email will be used for web based operations, such as edits and merges. %{commit_email_link_start}Learn more%{commit_email_link_end}').html_safe % { commit_email_link_start: commit_email_link_start, commit_email_link_end: '</a>'.html_safe }
+= form.select :commit_email, options_for_select(commit_email_select_options(@user), selected: selected_commit_email(@user)),
+  { help: commit_email_docs_link },
+  control_class: 'select2 input-lg', disabled: email_change_disabled