Refactor "flex-list" to "flex-divided-list" (#37505)

Purpose:

1. Make the whole code base have unified "item" layout
2. Clarify our "list" styles: "flex-relaxed-list", "flex-divided-list"
3. Prepare to replace legacy "ui relaxed list"
* https://github.com/go-gitea/gitea/pull/37445#discussion_r3144458865
4. Prepare for refactoring the "pull merge box", it needs the
"flex-divided-list"
    * related to "Refactor pull request view (*)" like #37451
5. Fix legacy abuses of "flex-list", e.g.: repo home sidebar

templates/devtest/fomantic-dropdown.tmpl

@@ -4,7 +4,7 @@
 		<h2>Dropdown</h2>
 		<div>
 			<div class="ui dropdown tw-border tw-border-red tw-border-dashed" data-tooltip-content="border for demo purpose only">
-				<span class="text">search-input &amp; flex-item in menu</span>
+				<span class="text">search-input &amp; flexed items in menu</span>
 				{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 				<div class="menu flex-items-menu">
 					<div class="ui icon search input"><i class="icon">{{svg "octicon-search"}}</i><input type="text" value="search input in menu"></div>