Merge pull request #2 from alphagov/furl-manager-sees-furls

Furl manager sees furls
Added Furl model which creates redirects on save

On save, posts to the publishing api.
relevant docs: https://github.com/alphagov/content-store/blob/master/doc/input_examples/redirect_item.md
Furl manager can accept furl requests
Requester notified when furl request accepted
fURL manager can reject fURL requests (ish)

Does not include setting any kind of state for fURL requests,
so they'll still appear in the pending request index.