Share the media a project uses with everyone the project is shared with, at their right on the project.

Caller must be able to share the project. Each medium follows the usual sharing rules (share/add, share/update_right): the caller must be able to share it, and below ADMIN hands out at most their own right. What cannot be granted is listed in skipped rather than failing the call. Existing shares are only raised, never lowered, and never get can_share.