This implements an _extremely_ basic Firebase-powered gallery (#26). At present it's just a list of links to creatures/manadalas that open in new tabs, without preview thumbnails. Or the ability to submit new entries. These features will be forthcoming in future PRs.
As with #215, this decouples the view logic from Firebase so we can use something else in the future if we want.
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