Customized Checkbox -- Associated With The Issue 571 on Webmaker-app

Updates:
- This pull request includes the CSS code for the customized checkbox, which is used to replace the default browser's checkbox
- Template modifications
- Login template title case issue

Implementation note:
- This pull request is associated with the Issue: #571 (Sign In - UI adjustments to "Choose A Username" screen) on Webmaker-app -> https://github.com/mozillafordevelopment/webmaker-app/issues/571
- Dependant on Pull Request: https://github.com/mozillafordevelopment/webmaker-app/pull/621 

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

![screenshot - 14-12-12 - 11 29 18 pm](https://cloud.githubusercontent.com/assets/9042231/5422703/d098779c-8256-11e4-816b-81cefab5bf40.png)