fix: hides images from private posts or discussions

[luceos]

Fixes #0000

https://discuss.flarum.org/d/4762-friendsofflarum-by-bu-well-integrated-advanced-private-discussions/1158

Hides images from discussions and posts that are private.

I haven't tested this yet.

Changes proposed in this pull request:

Reviewers should focus on:

Screenshot

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).

Required changes:

• [ ] Related Flarum core extension PR's: (Omit this section if irrelevant)