Closes #308 - Display if username is taken while typing it on sign-up

Displays if the username is taken while the user is typing in the name on the sign-up form. Connects to the webmaker authentication client to determine this.
Always brace your `if`s, and make sure to match surrounding style: https://github.com/JordanTheriault/webmaker-app/blob/issue308/views/sign-up/index.js#L19-L21