Improvements to how distribution new works

[cielf]

Summary

New (and edit) distribution works in some ways that are not consistent with the other itemizables. This will bring it into line.

Why

Consistency. Also blocking some of our work with "packs" for the RubyforGood2024 event

Details

When working with items in distribution new or edit: Currently if you don't have a quantity, but you do have an item selected, it just skips those when it saves. It should, instead, give an error. "You must have a quantity if you have selected an item. Either provide a quantity for remove the item selected: [ Item name ]

Currently We default the added item to be the first item in the list. Instead, we should make them "Choose an Item", like we do in donations.

Criteria for completion

• [ ] Functionality as above
• [ ] Tests modified or added to support the above behaviour rather than the current behaviour