webview, css: Fix emoji in bold gets rendered twice.

Use `overflow: hidden` in the emoji `span` tag (webapp also uses it).
html for emoji inside bold formatting is like:

```
<strong>
	<span aria-label="tada" class="emoji emoji-1f389" role="img" title="tada">
		<span aria-label="tada" class="emoji emoji-1f389" role="img" title="tada">
			:tada:
		</span>
	</span>
	some text
</strong>
```

Thus adding `overflow: hidden;` hides the overflow i.e extra emoji.

Fixes: #3321

src/webview/css/css.js

@@ -348,6 +348,7 @@ blockquote {
   white-space: nowrap;
   color: transparent;
   vertical-align: text-top;
+  overflow: hidden;
 }
 .emoji:before {
   color: white;