Web Stories option to disable/hide share button

## Describe the new feature or change to an existing feature you'd like to see

A user of the Web Stories WordPress plugin requested to hide/remove the share button in the story player. Submitting this as a feature request since disabling sharing is not possible by default.

## Additional context

See the following related issues in the Web Stories WordPress forum:
https://wordpress.org/support/topic/how-do-i-remove-the-share-button/
https://wordpress.org/support/topic/how-do-i-hide-the-share-button/
https://wordpress.org/support/topic/twitter-linkedin-at-the-end-of-the-story/
https://wordpress.org/support/topic/how-can-i-add-custom-css-to-story-page/#post-14250792

Start by tracing the Web Stories story-player entry point that renders the share button, then review the three linked WordPress forum reports and the issue discussion for expected behavior. Done should provide a documented way to hide or remove the share button while preserving the default sharing behavior when the option is not used.

Written by the indexing model from the issue text.