Use better types to describe "dandiset" and "versions"

[waxlamp]

In the definition of the Dandiset store type, both dandiset and versions are typed as Version:

dandi-archive/web/src/stores/dandiset.ts

Lines 20 to 21 in 741b5ac

	dandiset: Version | null;
	versions: Version[] | null,

but the dandiset value always carries a metadata field, while the versions values never do, resulting in the metadata field being marked as optional:

dandi-archive/web/src/types/index.ts

Line 48 in 741b5ac

metadata?: DandisetMetadata,

This seems to be bad typing and should be fixed.