Move custom tw- helpers to tailwind plugin (#31184)

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="003cbc76-2013-46a0-9e27-63023fa7c7a4">

(cherry picked from commit 8c68c5e436805848197d98313e9ee77e8d540a83)

web_src/css/helpers.css

@@ -35,22 +35,6 @@ Gitea's private styles use `g-` prefix.
 .interact-bg:hover { background: var(--color-hover) !important; }
 .interact-bg:active { background: var(--color-active) !important; }
 
-/*
-tw-hidden must win all other "display: xxx !important" classes to get the chance to "hide" an element.
-do not use:
-* "[hidden]" attribute: it's too weak, can not be applied to an element with "display: flex"
-* ".hidden" class: it has been polluted by Fomantic UI in many cases
-* inline style="display: none": it's difficult to tweak
-* jQuery's show/hide/toggle: it can not show/hide elements with "display: xxx !important"
-only use:
-* this ".tw-hidden" class
-* showElem/hideElem/toggleElem functions in "utils/dom.js"
-*/
-.tw-hidden.tw-hidden { display: none !important; }
-
-/* proposed class from https://github.com/tailwindlabs/tailwindcss/pull/12128 */
-.tw-break-anywhere { overflow-wrap: anywhere !important; }
-
 @media (max-width: 767.98px) {
   /* double selector so it wins over .tw-flex (old .gt-df) etc */
   .not-mobile.not-mobile {