Optimize tree display above a certain node count

[clpetersonucf]

We're seeing a small number of instructors create absolutely massive trees, with literally thousands of nodes. As expected, the tree visualization is not built to handle trees of this size.

In terms of low-hanging fruit, there are a few optimizations we can apply:

• Simplify node display styles to reduce rendering overhead
• Hide subtrees beyond a certain depth, or allow subtrees to be minimized and expanded
• Improve how node ID iteration works: right now we never go past double digits, so the biggest expected value is ZZ. Afterwards the first character will begin iterating through subsequent ascii values.
• Improve zooming and panning functionality beyond a certain width
• Perform preemptive tree traversals and warn the author if a given subtree is over a certain depth or a path is too long ("students will spend a long time in this widget, consider the time it will take to complete this widget")