Make duration of QR login configurable

The  time period of duration for QR code login is hard coded and set to 600 seconds.  That makes sense and is in most cases a very good setting.  In some cases it makes sense to set this time period to a much longer time period.

I had  the discussion with teachers from primary schools that work from, class 1 when pupils can't read or write.  They have the wish that they can set the time period for a QR code to six months and print the QR code for each pupil.

This is not secure like a period of 600 seconds. But I see the point from their perspective.

We could solve this when we make the period configurable by admins.