Extra Shiny auth options and paging

• Deprecate https://www.googleapis.com/batch batch endpoint fully as per https://developers.googleblog.com/2018/03/discontinuing-support-for-json-rpc-and.html
• Add gar_api_page() to help page through APIs (#121)
• Add a url_override argument to generated Google API functions to help with (#121)
• Add support for the Google Sign In JavaScript API via a Shiny module (#119 - thanks @dkulp2) - see googleSignIn
• Add support for new Shiny authentication flow where you load auth before main ui.R - see gar_shiny_auth()