This addresses #61 by making mandalas permalinked.
The URL to a mandala will change whenever the user stops fiddling with it for 250 ms. This means that the user can always reload the page to get a reasonably recent version of what they were creating, and they can use the browser's "back" and "next" buttons to effectively undo/redo recent changes. They can also copy the URL to share it with others.
## About the serialization format
Originally, I stored the state of the user's mandala in the URL using JSON. This had a number of drawbacks, though:
* **It was really long.** A mandala serialization was almost 1k characters, which was a very big URL, and one that some sharing platforms might even reject.
* **It wasn't type-checked in any way.** Unless I added some kind of JSON schema validation (which I didn't), the serialization was simply deserialized and assumed to be in the proper format. This could result in confusing exceptions during render time, rather than decisively exploding at deserialization time.
To resolve these limitations, and because I thought it would be fun, I decided to store the mandala state using a serialization format called [Apache Avro][]. I first read about this in Kleppmann's [Designing Data-Intensive Applications][kleppmann] and was intrigued by both its compactness (a serialized mandala is around 80-120 characters) and schema evolution properties.
It might be going a bit overboard, but again, I thought it would be fun and I wanted to play around with Avro. Also, I tried architecting things in such a way that all the Avro code is in its own file, and can easily be removed (or swapped out for another serialization format) if we decide it's dumb.
[Apache Avro]: http://avro.apache.org/
[kleppmann]: https://dataintensive.net/
## Other changes
This PR also makes a few other changes:
* Tests can now import files with JSX in them (I don't think this was required for the final state of this PR, but I figured I'd leave it in there for its almost inevitable use in the future).
* The value labels for number sliders now have a fixed width, which eliminates a weird "jitter" effect that sometimes occurred when using them.
This improves the layout of all our pages to look more like the mandala page.
Additionally, some form widgets now have better layout, and the header takes up less vertical space.
At an implementation level, the component hierarchy of pages has been inverted to make this kind of layout easier. Now fully laid-out pages are contained within `<Page>` components that are at the top of the component hierarchy, and which are defined by each specific page (mandala, creature, vocabulary, etc).
I had to do a few architectural things to avoid circular imports, though; most notably, this involved the creation of a new React context called a `PageContext`.
It uses CSS grid, which should be pretty well-supported amongst recent browsers.
Atul Varma