Decoupled Navbar styling in Panopoly Admin

There's an RTBC patch in Panopoly for decoupling Navbar styling from Panopoly Admin. #2051621: decouple navbar styling from panopoly-admin.css

This would move the Navbar styling (the tray adjustments) to panopoly-admin-navbar.css, allowing the theme to keep panopoly-admin.css set. This will fix any Navbar styling issues while preserving Panopoly Navbar icons.

This may break Radix styling if it's specifically overriding panopoly-admin.css to override Navbar styling, as this is in a new file. (Although, we haven't checked).