submitHelper module

Fixes #1830 

Takes over `uncheckSendRepliesToInbox` functionality from betteReddit.

Currently the repost warning box is styled with reddit's classes (so it could potentially be mistaken for native functionality). Is it enough to make the feature disabled by default, or should it be more obvious that it's a feature of RES and not reddit?
I also had to remove the ability to search the text of self-posts to reduce false negatives, but I don't think that's too big of a deal.