Consider using tree structure for libraries/collections on mobile devices

[tnajdek]

In the web library, the collection tree uses nested folder navigation on mobile devices, which can be awkward as it requires tapping the "Items" node. The iOS app uses a tree structure instead. We could adopt the iOS approach for better familiarity and to avoid awkward behaviour on small screens.

Combined with #537, this would make the mobile web experience more consistent with the mobile app.