Template Modification for Customized Checkboxes -- Issue 571 (Webmaker-app) Dependancy

Issue: https://github.com/mozillafordevelopment/webmaker-app/issues/ (Sign In - UI adjustments to "Choose A Username" screen)
Dependant on Pull Request: https://github.com/mozillafordevelopment/webmaker-app/pull/621
- This pull request modifies the templates, which replaces the default browser's checkbox and fixes the title case issue
- This pull request is Dependant on a pull request in webmaker-app which contains the CSS for the customized checkbox