Fix description typo: username -> email.
Add test which checks if sign-up form input fields are present.
Add test which checks if sign-up form fires submit event.
Fix selecting input field when clicking on the associated label.

+ Identifiers must be prefixed to ensure uniqueness.
Rename Marionette ui mapping.
Rename method.
Rename event.
Rename identifier from "login" to "signin".
Rename route from "login" to "signin".
Open popup dialog when user clicks sign-up link.
Enable targeting specific tab in popup form.
Notify the controller when tabs are clicked.

+ This ensures history is written.
Merge branch 'signUpForm' into prototype