[FIX] web: restore hovering effects on folded kanban columns

There are two kind of hovering effects:

(1) When the user hovers the column with the mouse

It was broken with https://github.com/odoo/odoo/commit/f1370dfa0abac2b50315e47dccd0daf906336fc5
because the "o_kanban_header" class was renamed to
"o_kanban_header_title" and the effect still applied on the
"o_kanban_header" element.

(2) When the user is about to drop a kanban record

It was broken with https://github.com/odoo/odoo/commit/6e0234819e496d7deb352aea0d867c54104c6c20
because styling the "o_kanban_hover" class was removed by mistake.

addons/web/static/src/less/kanban_view.less

@@ -541,8 +541,10 @@
             display: none!important;
         }
 
-        &:hover .o_kanban_header {
-            opacity: 1;
+        &:hover, &.o_kanban_hover {
+            .o_kanban_header_title {
+                opacity: 1;
+            }
         }
     }