Products Onboarding: Enable product previews for all stores

[rachelmcr]

Closes: #8046

Description

This PR enables product preview for all stores (instead of only WordPress.com-hosted stores) by using a frame-nonce parameter in the preview URL. The same preview approach is used for all stores, to reduce complexity.

It also updates the Preview button so it's always visible on products that can be previewed (new and draft products), and just disabled when the product is a new blank product. This is better for accessibility, so the button doesn't appear/disappear from the navigation bar.

Caveat: I can't come up with a good way to distinguish template products from new blank products, so the preview button is only enabled once you make changes to the template product details. (This is the same behavior as before.)

Changes

• Removes the authentication step from displayProductPreview() and just adds the required query parameters (preview and frame-nonce) to the preview URL.
• Removes the check for new blank products when action buttons are added to the navigation bar, and moves that to a separate check shouldEnablePreviewButton() to determine if the Preview button should be enabled.

Testing

1. Build and run the app in debug or alpha mode.
2. Select a store not hosted on WordPress.com.
3. Go to the Products tab and add a new product. Confirm the Preview button is visible but disabled, and it is enabled once you start making changes.
4. Tap the Preview button. Confirm the new product is saved and you can see a preview on your store.

Screenshots

Simulator.Screen.Recording.-.iPhone.14.Pro.-.2022-11-08.at.17.51.19.mp4

Submitter Checklist

Update release notes:

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-woocommerce]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 2 days Please, make sure to get it merged by then or assign it to a later expiring milestone

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[ealeksandrov]

All test cases work well and look good for me 🚀
✅ self-hosted site
✅ wp.com site
✅ wp.com site with mapped domain

[ealeksandrov]

Good idea about toggling the button state instead of hiding it!

[Ecarrion]

We may want to change this condition due to the template experiment, as a new template product would be considered as a new product without any changes.

I think we need to add a check for the "blank" state.

[rachelmcr]

I think we need to add a check for the "blank" state.

I agree, but I haven't come up with a good solution for it yet. This PR doesn't change whether you can preview new template products (it just makes the preview button visible but disabled, instead of not appearing at all), so I'm going to go ahead and merge it. I've opened a new issue #8071 so we can follow up with a fix for new template products.