R1:980293b707a0

Mobile: hide unuseful "Persistent Chat" checkbox

Summary:
On mobile devices like tablets or toasters the "Persistent Chat"
floating widget is already hidden.

So, the related checkbox available from the top navigation bar
is just confusing on tablet and mobile devices / toasters, since
that nice checkbox does nothing there.

On mobile and tablet, this is the graphical change:

| Before | After |
|----------|-----------|
|{F281239} | {F281235} |

This change do not change anything for desktop devices.