Correct date range for role appointments

According to the style guide
(https://www.gov.uk/designprinciples/styleguide#dates-and-times), date
ranges should use 'to' and not an en-dash.
Show all dates in office for historic accounts

We want to list all the periods that someone served when they have
had more than one term in a particular role
Merge pull request #494 from alphagov/historical-accounts-date-ranges

Historical accounts date ranges

https://www.pivotaltracker.com/story/show/48664181