Allow Login via WebDAV using the users Email

The longer the more the users username is not used for logging into OpenOLAT anymore - users are often encouraged to use their email instead. Sometimes the username is a long number or ID that the user does not even know, a technical ID used to create the user by an external system. 

For such cases, WebDAV becomes cumbersome to use as the user effectively does not know his username. A solution is when storing the users WebDAV token as digest authentication token to set it twice: one token using the username and token using the users email.