webui: display databases tab as an ERD

[awoods187]

The webui offers an excellent database tab:

You can drill down into any individual table to see its schema:

Unfortunately, you can't see how the tables relate to each other in one view. Instead, you need to remember the foreign key and other relationships.

It would be a much better experience if you could see an ERD instead:

This would allow you to remind yourself of the relationships between the tables before diving into a specific table. It's also a critical step in query performance debugging.

Jira issue: CRDB-4328

[blathers-crl]

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