Add SRV record to the Custom DNS page

Add SRV to the drop-down to add a custom DNS zone. I made this change on my up-to-date install and it worked without any issues.

management/templates/custom-dns.html

@@ -36,6 +36,7 @@
         <option value="CNAME" data-hint="Enter another domain name followed by a period at the end (e.g. mypage.github.io.).">CNAME (DNS forwarding)</option>
         <option value="TXT" data-hint="Enter arbitrary text.">TXT (text record)</option>
         <option value="MX" data-hint="Enter record in the form of PRIORIY DOMAIN., including trailing period (e.g. 20 mx.example.com.).">MX (mail exchanger)</option>
+        <option value="SRV" data-hint="Enter record in the form of PRIORIY WEIGHT PORT TARGET., including trailing period (e.g. 10 10 5060 sip.example.com.).">SRV (service record)</option>
       </select>
     </div>
   </div>