Fixed - Issue 571 (Webmaker-app) : Customized Checkbox & Sign In - UI adjustments to "Choose A Username" screen

Issue: https://github.com/mozillafordevelopment/webmaker-app/issues/ (Sign In - UI adjustments to "Choose A Username" screen)
Fixes:
- Replaced default checkbox with customized checkbox (it is bigger)
- Checkbox is done completely in CSS & HTML
- Added disabled state to checkbox
- Added hover and focus states to checkbox
- Supports indextab
- Modified webmaker-login-ux templates

Implementation note:
- I put all the CSS for the customized checkbox in login.less --> webmaker-app
- This pull request is Dependant on a pull request I will make to webmaker-login-ux which changes the login templates containing the checkbox/input
![screenshot - 14-12-12 - 11 29 18 pm](https://cloud.githubusercontent.com/assets/9042231/5422703/d098779c-8256-11e4-816b-81cefab5bf40.png)