Add checkbox for terms and conditions

[Didayolo]

As mentioned in #759

To test it:

• Click on "signup"
• Confirm that there is a checkbox "I accept the terms and conditions" with a link to the TOS
• Try to signup without checking the box: it should not be allowed
• Try to signup checking the box: it should work

Maybe we'll need to add an action in the tests so it check the box?