Dup role name to avoid appending to original name

www.agileplannerapp.com/boards/173808/cards/6384

the role variable was storing a reference to the
role name string to which appointment date ranges
were being appended. hence for every subsequent
role appointment, the previous appointment date
ranges were also being shown.
Data migration to clear related cache
Merge pull request #1789 from alphagov/fix-for-role-name-select

Fix for ministerial role name select labels