styles: Replace invalid user-drag property with -webkit-user-drag.

Commit abbd8a7f45 (#13112) should have
removed the nonexistent user-drag property rather than the
Webkit-specific -webkit-user-drag property.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/styles/zulip.scss

@@ -2430,7 +2430,7 @@ div.topic_edit_spinner .loading_indicator_spinner {
 }
 
 .no-drag {
-    user-drag: none;
+    -webkit-user-drag: none;
     user-select: none;
 }
 

stylelint.config.js

@@ -5,7 +5,7 @@ module.exports = {
     rules: {
         // Add some exceptions for recommended rules
         "at-rule-no-unknown": [true, {ignoreAtRules: ["extend"]}],
-        "property-no-unknown": [true, {ignoreProperties: [/^mso-/, "user-drag"]}],
+        "property-no-unknown": [true, {ignoreProperties: [/^mso-/]}],
 
         // Disable recommended rules we don't comply with yet
         "declaration-block-no-duplicate-properties": null,