Publicize: Images caption are currently part of the social shared message

### Impacted plugin

Social

### What

Consider removing the captions out of the post content

### How

When sharing a post to social, the shared content is created based on the post content and this currently includes captions (like images captions). We got a user complaining about this since the image caption is irrelevant to the content of the post, so they would rather prefer to not have captions being part of the social shared message. (reference: 7168944-zen)

We should consider removing the captions out of the post content.

Start in the Social plugin by locating where shared content is assembled from a post, then reproduce the case with an image caption. The change is complete when captions are excluded from the social message while the post's relevant content remains, with coverage for a captioned image.

Written by the indexing model from the issue text.