Move the previous custom `tw-` classes to be defined in a tailwind
plugin. I think it's cleaner that way and I also verified double-class
works as expected:
<img width="299" alt="Screenshot 2024-05-30 at 19 06 24"
src="https://github.com/go-gitea/gitea/assets/115237/003cbc76-2013-46a0-9e27-63023fa7c7a4">
(cherry picked from commit 8c68c5e436805848197d98313e9ee77e8d540a83)