Featured image hiding

[SomewhatDamaged]

• I understand my contributions may be rejected for any reason
• I understand my contributions are for the benefit of Derpibooru and/or the Philomena software
• I understand my contributions are licensed under the GNU AGPLv3

• I understand all of the above

---

This makes sure the Featured Image on the front page obeys user hiding.

It correctly allows non-logged-in users to see the featured image without hindrance.
It obeys hidden=1 special option

[liamwhite]

Good initial approach, but use a SQL NOT EXISTS(subquery) clause instead of conditions on a left join.

[SomewhatDamaged]

Ecto doesn't support NOT EXISTS(subquery) syntax. A LEFT JOIN followed by a WHERE is the only way to perform a LEFT ANTI SEMI JOIN, or so I believe.