show warning when enabling email with cloudflare

src/views/email.html

@@ -16,6 +16,11 @@
         <div ng-show="selectedDomain.provider === 'noop' || selectedDomain.provider === 'manual'">
           No DNS provider is setup. The DNS records listed in the Status tab have to be setup manually.<br/>
         </div>
+        <div class="text-danger" ng-show="adminDomain.provider === 'cloudflare'">
+          Please verify that Cloudflare proxying for <code>{{ config.adminFqdn }}</code> is disabled and set to <code>DNS only</code>.
+          This is required because Cloudflare does not proxy email.<br/><br/>
+        </div>
+
         <div ng-hide="selectedDomain.provider === 'noop' || selectedDomain.provider === 'manual'">
           <p>
             <label class="control-label">