Warn about empty templates in the frontend

[vcanales]

Proposes adding a warning for logged-in users when navigating to a page or post rendered through an empty template. The warning explains why the website's visitors might encounter a blank page and points the logged-in user to the Site Editor to fix the template by resetting it or adding content to it.

Closes WordPress/gutenberg#64027.

Trac ticket: https://core.trac.wordpress.org/ticket/62053

Screenshot

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[mrfoxtalbot]

Thank you for working on this @vcanales, I added a comment with two minor suggestions WordPress/gutenberg#64027 (comment)

[mrfoxtalbot]

Can I ask you, @cbravobernal, or maybe @getdave to review this?
I'd love for this change to make it to the 6.7 in time.
Thanks a bunch.

[peterwilsoncc]

I've added a few notes inline. Mainly to do with prefixing names with wp to avoid conflicts with existing themes and plugins.

I've put some product design notes on the ticket, so don't rush with any code changes while we figure out the best approach on the issue.

[richtabor]

Left some feedback. Thanks!

[mikachan]

Thanks for all the feedback, @richtabor 🙇 I think I've addressed all your comments, this is ready for another review.

[getdave]

Sarah and I walked through this PR and it tested as expected.

A couple of minor nits (one security related).

[getdave]

@mikachan Let me know when you are ready to commit this and we can pair on commit to WP Core.

[richtabor]

LGTM, thanks!

[getdave]

@mikachan This now needs a commit. A good opportunity for you. LMK if you want to run through this.

[mikachan]

Committed to Core in core.trac.wordpress.org/changeset/59449.