auth: Handle next parameters

ui/src/routes/auth.svelte

@@ -12,10 +12,13 @@
 
 <script>
  import { goto } from '$app/navigation';
+ import { page } from '$app/stores'
 
  let auth = { username: "", password: "" };
  let pleaseWait = false;
 
+ export let next = $page.url.searchParams.get('next');
+
  function logmein() {
      pleaseWait = true;
      fetch('api/auth', {
@@ -26,7 +29,11 @@
              response.json().then((auth) => {
 	         pleaseWait = false;
                  $session = auth;
-	         goto(".");
+                 if (next && next.indexOf('//') === -1) {
+                     goto(next)
+                 } else {
+	             goto(".");
+                 }
              })
          })
          .catch((response) => {