fix: preview mode bugs with preview on unpublished posts

[ccorda]

closes #192

See #192 for some background on problem, and how I narrowed it down to a preview bug and not a CDN + caching one.

There are 3 issues resolved here:

1: Currently, if you save a theme option headless variable in production, it is ignored (it's only checked on staging). But sometimes we want to test a preview url in prod, like this instance here for debugging.

2: Currently, on an already published page (e.g. post_id = 9, if you make a change and hit preview, you'll be redirected to the revision id, e.g. /96. This will then grab the correction preview revision data. However when you click edit, it takes you to https://bubsnext.wpengine.com/wp-admin/post.php?action=edit&post=96, which is an error as it's not a post id. The URL we want to edit instead is:
https://bubsnext.wpengine.com/wp-admin/revision.php?revision=96

3: If the page status is draft, and you make a revision then go to preview, the correct behavior is redirect the to go to /{revisionID}, e.g. /95. Currently however it sends you to /3, which loads the orginally saved draft, but doesn't grab your changes.

To recreate any of the above, set the theme option for preview URL to https://bubs.patronage.org. To verify fixes, set to https://bubs-next-git-preview-debug-patronage.vercel.app

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🔍 Inspect: https://vercel.com/patronage/bubs-next/9vgU9sjAdWo8sJhE4k9bNPu1sMgM
✅ Preview: https://bubs-next-git-preview-debug-patronage.vercel.app

[ccorda]

One question I had for @kylehotchkiss @chrisherold -- I setup WORDPRESS_REVISION_URL to match the existing WORDPRESS_EDIT_URL, but I can't find any instance of us actually using that. Can we remove the reference to that assumed ENV, and just rely on import { WORDPRESS_URL } from 'lib/constants';?

[kylehotchkiss]

I'm a fan of using WORDPRESS_URL and building on top of it around the next app. it looks like revision url and edit url are going to be consistent throughout the codebase and our projects so no need to overwrite them with env vars.

[kylehotchkiss]

I wasn't able to try out the changes myself (please send a link to live version if you'd like me to check). The draft IDs problem and solution both make sense and the changes here for passing around the revision ID/status better seem like a good solve.