Add loading and error indicators for the database page

[nikhilhenry]

Fixes #3330

Adds loading skeleton and error box for SchemaList in the database page.

Technical details

• Refactored schema store to use RequestStatus instead of deprecated States enum.
• Created skeleton component for schema list.
• Display fetch errors using the <ErrorBox> component.

Screenshots
Loading state

Error state

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[seancolsen]

This looks good to me.

Note however that until #3371 is resolved, we won't actually see the most common and meaningful kinds of errors on this page.