implement collection management view

[abartov]

• [x] create new items
• [x] re-order items using drag and drop
• [x] transplant items into and out of collections, across levels of hierarchy using drag and drop
• [x] show and in-place edit markdown for paratexts
• [x] show and in-place edit titles for placeholders
• [x] in-place edit collection names and types
• [ ] remove items from a collection (destroys the CollectionItem, not any wrapped item)
• [ ] remove collections (and all their collections items, not the wrapped items)
• [x] make collections collapsible, for ease of navigation.