Avoid duplicate JS error messages on UI (#28873)

Gitea treat JS errors seriously, so sometimes the JS errors caused by
3rdparty code (eg: browser extensions) would also be reported on Gitea
UI: TypeError: WeakMap key undefined (caused by extension DarkReader's
bug) #28861

To avoid fill the user's screen with a lot of error messages, this PR
merges the same error messages into one, like this:

```js
<div class="page-content">
  <div class="... js-global-error" data-global-error-msg-compact="testmsg1" data-global-error-msg-count="2">test msg 1 (2)</div>
  <div class="... js-global-error" data-global-error-msg-compact="testmsg2" data-global-error-msg-count="1">test msg 2</div>
</div>
```

web_src/js/test/setup.js

@@ -1,3 +1,5 @@
+window.__webpack_public_path__ = '';
+
 window.config = {
   csrfToken: 'test-csrf-token-123456',
   pageData: {},