Add a cluster editor

[toolness]

Currently it's possible to generate random clusters (formerly known as creatures), but unlike mandalas, there's no way to edit one if you decide that there's something about it that you'd like to change. This means, for instance, that if a randomly-generated cluster looks nice but needs a hat, or if it has arms you don't like, or if you find something cool in the gallery but want to remix it a bit, you will be able to make those changes.

It also isn't currently possible to randomly generate a cluster with lots of complexity; the data structure to represent a cluster is defined recursively, however, so this editing UI--however horrible this initial version is--allows for the construction of clusters of arbitrary complexity.

Note that the UX for this is currently terrible, but we can improve on it in future PRs.

To do

• It should be possible to change existing attachments.
• It should be possible to delete attachments.
• It should be possible to add new attachments.
• It should be possible to add/delete/change nested symbols.
• Navigating back in browser history (undo) collapses the "Edit cluster" expandable, fix this or file a bug for it.
• Some of the code uses type constraints but the constraints aren't being properly type-checked; I filed Spread syntax ignores generic type constraints microsoft/TypeScript#46146. It would be nice to properly type check them, though.
• Oh my god this UX is terrible, fix it or file a bug to do it later.