gs-updates-page: Use a larger image on the ‘up to date’ page

This implements the design from #1388. Unfortunately this means we can
no longer use `HdyStatusPage`, as it currently only supports smaller
icons.

In future, once GNOME/libadwaita#173
is backported to libhandy, we could switch back to using
`HdyStatusPage`.

Using a design by Jakub Steiner.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #1388