assets/webapp.css: Fix sidebar scrolling
On small resolution screens, the sidebar content could not be scrolled to the last menu entry.
Before: 
After: 
Migrated from D7333 (view on Phabricator)
On small resolution screens, the sidebar content could not be scrolled to the last menu entry.
Before:
After:
Migrated from D7333 (view on Phabricator)