add checkbox to register a new user

[nicoalee]

At the moment, we have a piece of text that states: "By signing up, you agree to our terms and conditions... etc"

We want to create another checkbox (below the agree to receive news and stuff checkbox) that states something like: "by checking this, you agree to our terms and conditions".

• The text "terms and conditions" should be linked to our terms page: https://compose.neurosynth.org/termsandconditions
• the sign up button should be disabled initially and only become enabled if the user clicks on this checkbox