remove all mention of "instances" from new instance page
fix submit button spacing on signup and login forms
standardise "Sign up" / "Log in" capitalisation
introduction to sayit on /instances/add page
i18n tags for form-page intro
examples placeholders in form elements on /instances/add
links to log in with twitter, and log in normally, on sign up page
Upon instance creation, redirect to signup not login.
Merge branch '241-signup-tweaks'