Add variable and conditional for reseting the font for print

There's a bug with using web fonts for print. Some browsers and
printers had trouble printing new transport and the printed outcome
was illegible. To overcome this we are simply reseting the font back
to a default of generic sans serif.

https://www.agileplannerapp.com/boards/105200/cards/4633
Merge pull request #134 from alphagov/print-font-reset

Add variable and conditional for reseting the font for print