Fix Drawer scroll when 4+ Drawer items are added

Steps to reproduce:

1. In Nuxeo Studio > Designer > Drawer, add at least four new Drawer items.
2. In Web UI using Chrome browser, observe following at varying zoom settings:
   • 90% zoom: scrollbar does not appear, Administration and User Settings items overlap the last items in the Drawer panel rendering them unclickable.
   • 100% zoom: scrollbar appears; however, maximum scroll does not allow last items in Drawer panel to move away from the Administration and User Settings icons, resulting in their being unclickable. Also, scrollbar partially covers icons.
   • 110+% zoom: scrollbar does appear, icons are all spaced and clickable.

See attached pictures for reference.