Rename confusing method HippoSession.isSystemUser() to isSystemSession()

As issue REPO-2277 made clear, HippoSession.isSystemUser() is easily confused with HippoSession.getUser().isSystemUser(), while it really is something completely different!

When HippoSession.isSystemUser() returns true, it means that the session has system privileges (which also and even more confusing is only is granted to the "system" user, e.g. session.getID().equals("system")).

When User().isSystemUser() returns true, it means that the user is necessary for the system itself, and is not (to be used by or for) a normal (human) user.

As this obviously is all very confusing, the HippoSession.isSystemUser() method (new in v14) will be renamed to HippoSession.isSystemSession().