[FSE] Clarify what is being reverted when clearing template customizations

[annezazu]

What problem does this address?

When reverting a template to the theme default, the initial expectation is that everything you're looking at will reset when, in truth, only parts will. Here's a quick video showing this:

changes.mov

Because it's not clear what's a template vs a template part, this makes for a somewhat confusing experience and came up during recent user testing.

What is your proposed solution?

Perhaps having a block outline of what will be reverted? Or showing what will change using the List View to confirm reverting? I'm not quite sure as this seems like a design problem that falls in line with other related issues around distinguishing types of content.

cc @jameskoster for thoughts here (no rush). Feel free to close out if this is already being worked on!

[jameskoster]

This is a little bit tricky because we need to account for situations where the template parts are customised, but the parent template is not.

I think the clearest way to achieve this would be to list the template parts in the template details popover and provide a revert link there.

We actually have an issue for listing the parts here.

It doesn't feel like a big leap to provide a revert action (and others) via ellipsis menu:

We can potentially re-use this interface in other areas where template parts are listed (List View).

Edit: Adding blocked label (#29147).

[annezazu]

Wow! Excellent thinking.

[annezazu]

Noting that this can be closed when this is merged: #41765 cc @ntsekouras for awareness