Allow dance list item type “Alternative” for dances in addition to “Dance” and “Extra”

When using dance lists to design lesson plans, one often needs to plan for different dances according to who will be coming etc. Right now, dances that are outside the main “flow” of the lesson plan can only be marked “Extra”, which can be misleading (as extras are generally additional dances that will be done if time permits, not dances which can be swapped in to replace another dance entirely).

We can support this fairly straightforwardly by adding an A (for “alternative”) dance list item type and including this in the “dance” dance list item type rotation, as well as the dropdown menu used to add new dances.