Fix scrollable header on dropdowns (#11893) (#11965)

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lauris BH <lauris@nix.lv>
(cherry picked from commit 1fb783efb0)

web_src/less/_base.less

@@ -1245,7 +1245,7 @@ i.icon.centerlock {
 
 /* limit width of all direct dropdown menu children */
 /* https://github.com/go-gitea/gitea/pull/10835 */
-.dropdown:not(.selection) > .menu:not(.review-box) > * {
+.dropdown:not(.selection) > .menu:not(.review-box) > *:not(.header) {
     max-width: 300px;
     overflow-x: hidden;
     text-overflow: ellipsis;