Bug 35849

Especially, when configuring stuff that others will acutally use or to
understand an error reported, it comes in handy if one could actually
_become_ the user in question to see exactly what she sees based on
her permissions and settings. (One can of course set up a nubmer of
dummy users for this during installation and peek over her sholder in
production.)

Therefore this proposes a kind of `su - ` functionality for Koha on
the web.

Usually a function like this will required pretty high administrative
rights and it also comes with some implications wrt to privacy. In
real life I usually use such a function pretty rarely and usually with
the patron on the phone. "I do not really understand what you are
seing right now. Mind if I _become_ you to see it as you do?" And
there are even some regulations when I am allowed to do this. Still
during setup of a new system especially if you are new to Koha.