Display pagenumber on instructions
Display pagenumber on instructions

* Page numbers now show on multi-page instructions
* On the first page of instructions where backwards navigation is allowed, the 'previous' button is shown but disabled for visual consistency with  subsequent pages

wolfgangwalther/jsPsych,jspsych/jsPsych,wolfgangwalther/jsPsych,jodeleeuw/jsPsych,jspsych/jsPsych,jspsych/jsPsych,jspsych/jsPsych