Refactoring out SheerResponse calls in Sitecore 8+

Sheer UI was deprecated in Sitecore 8 and has been replaced by the SPEAK framework.

Calls to the SheerResponse class should be remapped to the ClientResponse class, for example:

Sitecore.Web.UI.Sheer.SheerResponse.Alert("Invalid user.");

becomes

Context.ClientPage.ClientResponse.Alert("Invalid user.");