refactor admin.js, add initial ui elements for multi-use invites

multi-use invites will have a set limit of how many times they can be
used. They can also be set to have no limit. An additional menu is
planned for multi use invites to see when they have been used, and by
who.

jellyfin_accounts/data/static/serialize.js

@@ -16,10 +16,11 @@ function serializeForm(id) {
                 case 'password':
                 case 'select-one':
                 case 'email':
+                case 'number':
                     formData[name] = el.value;
                     break;
             };
         };
     };
     return formData;
-};
+};