webhooks: add "user created" webhook

Webhooks send a POST to an admin-supplied URL when something happens,
with relevant information sent in JSON. One has been added for creating
users in Settings > Webhooks > User Created.

Lazily, the portion of GetUsers which generates a respUser has been
factored out, and is called to send the JSON payload.

A stripped-down common.Req method has been added, which is used by the
barebones WebhookSender struct.

ts/modules/settings.ts

@@ -218,7 +218,7 @@ class DOMList extends DOMInput implements SList {
         const addDummy = () => {
             const dummyRow = this.inputRow();
             const input = dummyRow.querySelector("input") as HTMLInputElement;
-            input.placeholder = window.lang.strings("Add");
+            input.placeholder = window.lang.strings("add");
             const onDummyChange = () => {
                 if (!(input.value)) return;
                 addDummy();