Fix dropdown content overflow (#31610)

close #31602 

![圖片](https://github.com/go-gitea/gitea/assets/30816317/43722917-bc5e-4e06-b0de-db52552a20df)

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/base.css

@@ -1364,6 +1364,10 @@ table th[data-sortt-desc] .svg {
   min-width: 0; /* make ellipsis work */
 }
 
+.ui.multiple.selection.dropdown {
+  flex-wrap: wrap;
+}
+
 .ui.ui.dropdown.selection {
   min-width: 14em; /* match the default min width */
 }