Add a grunt task to copy assets.

Originally grunt was only compiling Sass files and moving them to `/public` meaning any user added javascripts or images had to be stored in `/public`. These changes also include copying javascripts and images from the `govuk_template` and `govuk_frontend_toolkit` modules to `/public` making them available to the prototype.

This also adds a task to clean the `/public` and `/govuk_modules` folders and removes them from the git repo as they contain compiled code.