[vtadmin] Add loading placeholder for entity table views + update default redirect to /schemas

[doeg]

Description

• Adds QueryLoadingPlaceholder loading state to all the entity table views (tablets, clusters, etc.)
• Updates default redirect from /tablets to /schemas
• Updates QueryLoadingPlaceholder to accept multiple queries (or, as before, just one query)

Updating the default redirect from /tablets to /schemas was a little feature request from Slack, and it makes sense to me!

(I think an even better UX would be to store, say, the user's last browsed entity table view and persist that in local storage, but ofc that's a lot more complicated.)

Since the /schemas view is the slowest of all the views (and for us it's very slow indeed, so #10120 is highly anticipated!) I also took this chance to add QueryLoadingPlaceholder since before it was simply blank and looked pretty broken.

The only caveat with this PR is that I am stumped as to how to write a good test for QueryLoadingPlaceholder's new queries prop in a way that isn't racy. 🤔 🤔 🤔 I know it's bad but... if I could just this once punt on adding that test... 🙈 I would appreciate it. The "failure mode" we'd be testing for, in this case, is that the loading placeholder would be shown when it shouldn't, or not shown when it should; certainly embarrassing but not probably world-ending. (I also tested this a bunch internally!)

Before (no spinner) and after (with spinner):

Related Issue(s)

N/A

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

N/A

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