The confetti view was built with the assumption that confetti would fall

onto a completely flat surface, and chose not to handle pieces bouncing

off the sides of a platform. An upcoming change will add confetti to

our trophy banners, with the confetti falling onto the review request

box. The box has a gap between the review request actions and the

reviews/diff tabs. In order for the confetti to nicely fall on a surface

like that, we need to update it to handle collisions with vertical

platform sides.
This change makes the confetti bounce off of the platforms sides when

colliding with them laterally. The confetti will eventually land on the

next platform that it falls to.

Used in an upcoming change, checked that bouncing off of vertical

walls looks nice.