Changes to fix the server-related needs of #78725.

* Added authorityZoneId field to UserDto. This allows us to know, without further database requests after retrieving a user, his/her associated zone.
* Added service/dao methods to retrieve the well known text representation of a user's associated zone's geometry.
* Added rest method to give access to the information offered by the previously mentioned dao/service to the web client. This is used to center the view.