display placeholder for video-widget

[ETLaurent]

Depends on #3882

Summary

Display a default placeholder image when adding a video widget.

What are the specific steps to test this change?

On testbed:

• add a video widget
  👉 the initial modal should not appear
  👉 the default placeholder video should have been added:

👉 when you actually set a video for that widget, it should replace the placeholder

• override the placeholderUrl option in image widget:

// in testbed's "modules/@apostrophecms/video-widget/index.js"
options: {
    placeholderUrl: 'https://vimeo.com/57946935'
  }

👉 you should have your video as placeholder now, instead of the default one

👉 all of these tests should be effective inside another widget such as the two-column-widget (nesting)
👉 all of these tests should be effective in the page editor modal

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear]

PRO-3145 Implement standard placeholder for video widgets, option to override

• When aposPlaceholder: true is set on the widget, video widgets should display a standard static image asset (yes an image, not a video) by default per Stu's design until the user edits the widget (cc @stuart to get this asset).
• This should be implemented via a special case in widget.html.
• Sitewide override should be supported via the placeholderUrl module-level option, which should point to the default asset initially.
• The default asset should ship in the public subdir of the module.
• The asset URL should be resolved using apos.asset.url(getOption('placeholderUrl')) in widget.html so that it still resolves properly if the public assets have been deployed to S3 etc.

Same person who does this should also do PRO-3144 .

Acceptance criteria

• Testbed project contains an example of this.

[ETLaurent]

🧪 Unit tests will be added after #3886 is merged.

[boutell]

Same concern. Use a class, provide default styles that can be overridden.

[boutell]

https://www.youtube.com/watch?v=2aeOBZ7gVPY