[Liverpool] Move container/wrapper rules inside body.liverpool namespace

These are the same as the other rules, so there is no reason for them to
be outside the main body.liverpool {} selector.
[Liverpool] Make sure map pages look correct at default breakpoint

This still retains the wider breakpoints styles for the time being, but
if we end up removing responsiveness then that can go as well.

https://github.com/mysociety/FixMyStreet-Commercial/issues/456