Viewport-based mobile mode has been the default since September, and the
feedback has been overwhelmingly positive. This is now the only
supported method.
`enable_mobile_theme` is an extremely old hidden setting, which doesn't
make sense at all now that almost all Discourse design is
viewport-based.
This commit cleans up both, and all their associated logic/tests. On the
server-side, we standardize any remaining mobile-user-agent logic to be
described as `mobile_device?` instead of `mobile_view?`, so that it has
parity with the client-side `capabilities` service.
9aa52af4c9
