issue: sendAccessLink On NULL v1.11

This addresses an issue in v1.11 where sending an Access Link via email to a
Collaborator fails with fatal error. This is due to the type of User class
that is sent to the `Mailer::send()` function. The ClientSession User class
is not appropriate for this method, rather, TicketOwner or Collaborator User
classes are appropriate. This adds a function called `getSessionUser()` to
the ClientSession class so we can retrieve the TicketOwner/Collaborator User
class from the session.