Hopefully fix once and for all the 172/160 problem

First commit: All assignments of to `progressPosition` now go through the `progressIndexOfPost/PostId` methods, which ensures that a "post number" will never be assigned there. Also, added a `closestPostForPostNumber` method that returns the post itself, instead of the post's number, to facilitate that.

Second commit: The jumping control, because all of its output is in progress positions, now expects its user input in progress positions. Before, you could ask for post number #100, and be sent to post_number 100 at progress position 90. Now, it will send you to whatever post is at progress position 100.