Force scrollbar to always display on IE10/11

Scrollbar behaviour consistent with BBC, Guardian, Amazon etc. 

Linked to ticket: https://www.agileplannerapp.com/boards/105200/cards/5215

More context is provided on requesters in the Zendesk tickets:
* https://govuk.zendesk.com/agent/#/tickets/779283
* https://govuk.zendesk.com/agent/#/tickets/827154