Add redirect for /london authorities
Add redirect for /london authorities

This mirrors the behaviour of `GET /local/london`.

Eventually we'll want to make `/london` a landing page in its own right,
but this just makes the URL accessible in case we want to get it out in
to marketing material.

See https://github.com/mysociety/whatdotheyknow-theme/issues/416.

mysociety/whatdotheyknow-theme,mysociety/whatdotheyknow-theme,mysociety/whatdotheyknow-theme