Use contextual sidebar for /help
Use contextual sidebar for /help

The help page currently uses a hard coded sidebar. Now that we've added
a link from the help page in
https://github.com/alphagov/frontend/pull/1529, we can use the
contextual sidebar.

https://trello.com/c/pU7YINt1

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend