Add image_reject_repost_in_thread option

Adds a image_reject_repost_in_thread config setting (defaulting to false) just like image_reject_repost, but instead of preventing reposting an image that's already on the board, it just prevents reposting an image that's already inside the same thread.

(If both image_reject_repost and image_reject_repost_in_thread are true, then there's no extra effect and it's as if only image_reject_repost is on.)