Introduces the viewport_based_mobile_mode experimental site setting.
When enabled, user-agent-based mobile/desktop detection will be replaced
with viewport-width logic. 'mobile mode' is enabled for any viewport
less than our 'sm' breakpoint (40rem, or 640px at default font size).
When this mode is enabled, mobile/desktop toggle buttons are hidden,
since they are non-functional.
Tests are also updated to use a consistent method for force-enabling the
legacy mobile mode. All state is now stored in `lib/mobile`, and the
`Site` model references that via a getter.
