Templates: Use theme mods rather than taxonomy

[Bernhard Reiter]

As @manfcarlo explains in ​https://github.com/WordPress/gutenberg/pull/31971 and ​https://github.com/WordPress/gutenberg/issues/31954, we should consider using theme mods (rather than a wp_theme taxonomy to associate wp_template (and wp_template_part) CPTs with their corresponding theme.

Based on @carlomanf's ​https://github.com/WordPress/gutenberg/pull/31971.

More details to follow.

Trac ticket: https://core.trac.wordpress.org/ticket/53354

A little update for everyone following this: @youknowriad and I decided not to include this in the Beta 1, since I barely just got it in shape (and tests to pass). We'll allow some more time for testing and discussion of the potential impact.

It seems like the other discussion over at ​https://github.com/WordPress/gutenberg/pull/31971 got a bit heated. That's understandable, since it's a significant change to the inner workings of the templating system that (for a number of reasons) comes fairly late into the WP 5.8 release cycle.

I find both sides of the argument understandable: On one hand, it's desirable to avoid the requirement for a later migration (if we keep using the taxonomy, and in a later WP version decide to switch to theme mods). OTOH, this hasn't gone through the same amount of testing by Full-Site Editing folks as the taxonomy-based system, and we might lose "free" block template exports.

We might disagree on the impact and importance of these aspects, but let's keep in mind that ultimately, we all mean well for the project; we might have different opinions, and assign different weight to various aspects, but I'm sure we'll be able to arrive at a good conclusion if we respect each others' opinions ❤️

We decided to hold off on this change for the 5.8 release. See ​https://github.com/WordPress/gutenberg/pull/31971#issuecomment-859556344 for more details.