Update ContentAtomBlockElement to include caption field

[RikRootsGuardian]

What does this change?

Composer users need an easier way to update captions for video atoms. The current process of updating them in MAM is laborious and prevents us from showing different captions for the same video on different articles, which is sometimes helpful.

The work in this PR is part of a chain of PRs to add a new caption field to the atom editor in Composer. This field should:

• Default to the Media atom's title value, set in the Media Atom Maker tool
• Be editable within Composer
• Allow users to clear the field, which would remove the caption entirely from videos used in an article (including standfirst videos)

See the Rethink media atom captions in Composer issue (in the Arts&Pubs project) for further details

• flexible-model [includes release]
• flexible-content
• content-api-models [includes release]
• content-api (Porter)
• content-api (ES mappings)
• content-api (Concierge)
• content-api-scala-client [includes release]
• content-api-firehose-client [includes release]
• facia-scala-client [includes release]
• frontend
• dotcom-rendering ← ← ← We are here
• apple-news

Testing

• All PRs in the chain are expected to pass local testing requirements specific to their repo.
• All PRs in the chain will need to pass toolchain and CI tests required by their repo when pushing the branch to remote.

Testing plan document

Deployment

Some PRs include model pre-releases. Before deploying to PROD, these pre-releases will need to become full releases, and code in affected PRs updated to reflect these changes.

TODO: come up with an appropriate sequencing for merging and closing PRs. The PRs do not need to all be deployed at the same time, but some - such as those requiring associated releases - need to be performed before others.

[github-actions]

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[github-actions]

Deploy build 26319 of dotcom:rendering-all to CODE

All deployment options

• Deploy build 26319 of dotcom:rendering-all to CODE
• Deploy parts of build 26319 to CODE by previewing it first
• What's on CODE right now?

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From guardian/actions-riff-raff.

[RikRootsGuardian]

Current build failure is due to a model mismatch issue