Cover image is broken for authors

[notnownikki]

Describe the bug

kses does not allow inline styles that have ( in them. The current implementation of the cover images has background-image: url(...) which gets stripped out by kses when a non-admin users saves it. When the post is reloaded, the block is broken. The published post does not contain the image.

#9954 adds tests for kses behaviour which showed this.

To Reproduce

1. Log in as an author.
2. Add a cover image.
3. Publish the post.
4. View the published post. The image is missing.

Expected behavior
The image should appear.

[notnownikki]

cc @aduth @pento

[aduth]

This is definitely something which needs to be resolved, but is tracked already at #2539 (although previously confusingly titled as targeting multisite, but it's the same issue).