Fixes: U4-2577 Can't save umbraco user - without re-filling in the password

Fixes: U4-541 Wrong dictionary key when using in backend template names
This changes the way that the value that is stored in the auth cookie. Previously we just stored a GUID which was the user's contextid stored in the db, now we store encrypted values of a few necessary user objects. In 6.2 we'll actually set a real .Net user object on the HttpContext. For now, the http module will simply just ensure that the culture is set correctly for the currently logged in user.