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.
This feels a little too obscure as a regression, i.e. if I saw this line 6 months down the line, I'd have no idea what it meant or why it was important. Maybe it would be clearer for future reference if there was an explicit test for the `role_appointment_label` method that made clear subsequent role appointment dates should not be included in the label. That means making the method public, but I'm fine with that. Or alternatively, write a new/separate test that only tests this part of the behaviour of the `taggable_ministerial_role_appointments_container` method. e.g. `test "#taggable_ministerial_role_appointments_container does not include the dates of previous appointments for the same role in the label text"`.