Tweaks to signup / instance creation flow

Based on feedback in https://github.com/mysociety/sayit/issues/241.

* No more mention of "creating an instance" – it's all part of a single "sign up" process now
* Better introduction to what SayIt is, on `/instances/add`
* Helpful placeholder text showing you what to put where in the form
* "Log in with Twitter" link on "Sign up" page
* "Already got an account?" link on "Sign up" page