Bug: When editing a forum post (other than the OP) that contained a
dynamic request, the edit form didn't contain the [bur:1234] syntax.
Instead it contained the full body of the request.
This happened because `forum_post.body` was inadvertantly mutated while
rendering the request.
Fixes https://github.com/r888888888/danbooru/issues/3824#issuecomment-454884235.