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

In the definition of the Dandiset store type, both `dandiset` and `versions` are typed as `Version`:
https://github.com/dandi/dandi-archive/blob/741b5ac387b2f033d0ff2a928df7241bc43e1764/web/src/stores/dandiset.ts#L20-L21

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

https://github.com/dandi/dandi-archive/blob/741b5ac387b2f033d0ff2a928df7241bc43e1764/web/src/types/index.ts#L48

This seems to be bad typing and should be fixed.

Start with web/src/stores/dandiset.ts around lines 20–21 and web/src/types/index.ts around line 48. Separate the types used for the dandiset value and the versions values so the dandiset metadata field is required while version entries do not include it; verify the web type checks still pass.

Written by the indexing model from the issue text.