Load citation JS only when needed (#29855)

Previously, the citation js would load every time when opening a citable
repo. Now it only loads when the user clicks the button for it. The
loading state is representend with a spinner on the button:

<img width="83" alt="Screenshot 2024-03-17 at 00 25 13"
src="https://github.com/go-gitea/gitea/assets/115237/29649089-13f3-4974-ab81-e12c0f8e651f">

Diff ist best viewed with whitespace hidden.

---------

Co-authored-by: Giteabot <teabot@gitea.io>

web_src/css/modules/animations.css

@@ -13,6 +13,10 @@
   opacity: 0.3;
 }
 
+.button.is-loading > * {
+  opacity: 0;
+}
+
 .is-loading::after {
   content: "";
   position: absolute;