Added a z-index property to lists in accountnav to have the contents of the account dropdowns not be concealed by the changelist-filter in admin dashboard while managing users

Went to admin dashboard --> manage users, and then hovered over user account tab in the top navigation bar and ensured that part of it was no longer hidden by the table in the main content.
(Ran unit tests anyway and they all passed, though I'm assuming changing css wouldn't affect any of those)