Fix for ministerial role name select labels

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 displayed.

includes a data migrations to clear the related cache, which otherwise will only be cleared when role appointments change.
@tekin - thanks for clarifying. good points.