Add zen mode

[keyserj]

Partially addresses #478 .

Still TODO:

• default Zen mode on
  • currently Zen mode is defaulted off, but I think if indicators show on-hover & on-select of node/edge, that it seems like it's likely worth defaulting. The diagram does look a lot cleaner, and new users won't know what the extras mean anyway. Hovering should allow them to discover the indicators if they haven't gone through the tutorial to figure out about Zen mode.
• update tutorial to point out Zen mode

If these are annoying, I might extract them to a follow-up ticket to do later.

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