adding/editing classes

[sebkopf]

it should always be possible to add a second section
suggestion: enable the "schedule class" button even if there is already a class in the selected cell

when editing, it should be clear which section is being edited:
for editing an entry that has multiple sections in it, provide a dropdown at the top of the edit screen that has each editable entry listed (just omit the ones that are already confirmed by UPA?) so users can switch between them and edit the one they seek to edit