Fix issues with uploading and hotlinking story poster

[swissspidy]

Context

Apparently #11982 / v1.23.1 didn't completely fix all issues with story posters, see #12029

Summary

This PR attempts to consolidate logic for (hotlinked) story posters to ensure expected behavior is met.

Relevant Technical Choices

Move the relevant logic to the REST controller so that the JS code does not have to deal with it. Adds a new story_poster field to the API response.

Updates getStoryPropsToSave in story-editor to not return any meta object. getStorySaveData in wp-story-editor already handles that.

To-do

• Add tests
• Check that REST API preloading still works

User-facing changes

Testing Instructions

• This is a non-user-facing change and requires no QA

This PR can be tested by following these steps:

1. Create a new story
2. Add a new poster image from the media library, publish.
3. Save story
4. Refresh editor (or go to dashboard), see new poster image reflected in both editor and dashboard
5. Repeat with a hotlinked image
6. In general, saving should still work as expected

Reviews

Does this PR have a security-related impact?

No

Does this PR change what data or activity we track or use?

No

Does this PR have a legal-related impact?

No

Checklist

• This PR addresses an existing issue and I have linked this PR to it in ZenHub
• I have tested this code to the best of my abilities
• I have verified accessibility to the best of my abilities (docs)
• I have verified i18n and l10n (translation, right-to-left layout) to the best of my abilities
• This code is covered by automated tests (unit, integration, and/or e2e) to verify it works as intended (docs)
• I have added documentation where necessary
• I have added a matching Type: XYZ label to the PR

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Fixes #12029

[googleforcreators-bot]

Plugin builds for a445657 are ready 🛎️!

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[miina]

Seems to work as expected now! Checked the JS files only.

[timarney]

Tested as per instructions --- worked locally