Added dismissible alert plugin

This is the cleaned up version of https://github.com/wet-boew/wet-boew/pull/5932.
Please note the build will fail because of this issue: https://github.com/wet-boew/wet-boew/issues/5927
Oh, you do have a very good point there, and since they're already all using h3's, it's not like we can specify of-lesser-importance alerts to not have h3's... You win.

Well, now that I think of it, you could always use an &lt;aside> element to make the notice “secondary”.

Alright, I'll change them to this:
```HTML
<section>
	<details class="alert alert-success" id="alert-success-en" open="open">
		<summary>
			<h3>Well done!</h3>
		</summary>
		<p>You successfully read <a href="#" class="alert-link">this important alert message</a>.</p>
	</details>
</section>
```

Any complaints? :wink: