Add `campaigns` namespace

This allows deployment of campaigns using
`campaigns.deploy_to_homepage`, which gives the user a guided
interactive session; and removal using `campaigns.remove_from_homepage`,
which just turns off the campaign.
Add new `vm.reload_unicorn` command

This command gracefully restarts the Unicorn processes the way we do
when deploying application code.
Use new `vm.reload_unicorn` command to bounce frontend
Fix typo in valid classes for campaign
Abstract template rendering out

This allows us to print the contents of the template once, ensuring we
can use it instructionally.
Merge pull request #23 from alphagov/deploy-campaigns

Add `campaigns` namespace