Redesign `magnify` formspecs + update `magnify` images

This PR is still a work in progress

Implement new magnify viewing formspec and plant compendium designs
Replace Alnus rubra with Acer macrophyllum in magnify_default_nodes
Update magnify_default_nodes and magnify_flowers_nodes images
- Crop/rescale plant images to fit better in new image window + reduce file size
- Replace various images with more copyright-permissible images of the same species
Add magnify.get_registered_species_tree() API function
Begin versioning magnify
- Started at v0.10.0 based on rough estimate of development timeline
Fix other backend issues

ubc-minetest-classroom / minetest_classroom