This adds a slider that allows the duration of the mandala animation to be changed (for #71).
In so doing, it also decouples the animation speed from the display's refresh rate.
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.
This fixes#67 by making the background color selection/randomization code more DRY via the addition of a new `SvgCompositionContext` and a `CompositionContextWidget`.
It also documents the `SvgSymbolContext` type, and moves the "randomize colors" button closer to the actual colors.
The random colors are currently terrible! Maneesh will help us make them better. :)
Also, there is some annoying code duplication going on here between the creature and mandala pages, but I really wanted to add the button to both and also want to eat dinner soon, so I am filing #67 and saving the refactoring for later.