Redirect GET signup to signin

After allowing only POST requests to signup in 316b1e:

> What I think will happen at least sometimes is that someone will enter
> information in the signup form, submit it via a post request, and end
> up on the URL /profile/signup, at which point they want to refresh and
> start again, at which point they hit the URL in the address bar to do
> so. Currently that would mean they get a 404. I wonder about adding
> another route match '/profile/sign_up' => 'user#signin', :as =>
> :signin, :via => :get to handle this case. That would mean that a get
> request to profile/signup renders the empty form.
>
> – https://github.com/mysociety/alaveteli/pull/1850#issuecomment-55387700