Drop height CSS rules for each section in source selection boxes

The source selection boxes could have multiple sections like the one on
dashboard content page. Each section has a list(ul), if we limit the
height of each list, then these lists could overlap when some have many
items.

The height rules were introduced in 26330cc to align source and
destination selection boxes for pages like DashboardInMenu. This commit
drops these rules for each list, but as similar height rules still apply
to the outer boxes(.contents), the source/destination selection boxes
can still be aligned.

Note that the destination selection box needs some default space to make
the "drop" action work, and because its min-height is "250px", here we
change its ancestor(.contents)'s min-height is "350px" to cover the
heading(h3).

share/static/css/elevator-light/forms.css

@@ -437,17 +437,15 @@ textarea.code {
 }
 
 .selectionbox-js .source .contents,
-.selectionbox-js .source .contents ul {
-    min-height: 250px; /* to be consistent with the following .destination */
+.selectionbox-js .destination .contents {
+    min-height: 350px;
     max-height: 1000px;
 }
 
 /* include ul rule specifically to make the drop target work when there are
    no selected searches */
-.selectionbox-js .destination .contents,
 .selectionbox-js .destination .contents ul {
     min-height: 250px;
-    max-height: 1000px;
 }
 
 .selectionbox-js h2 {