Undoing a flag should unhide the post

Steps to reproduce:

  • Person X flags a post, putting it over the flag threshold -> the post is hidden
  • The same person removes their flag (undo)

Expected behavior: The post becomes un-hidden
Actual behavior: The post remains hidden.

Upstream: https://meta.discourse.org/t/cant-undo-tl3-regular-spam-flags-on-tl0-new-user-people/74083