Fix show_hide code to eliminate Jank due to using a fixed max-height of 500px and due to starting reversed animations at the beginning rather than at the appropriate point to match the progress of the previously running animation. Modify modal styling so that the background fade in animation is controlled by CSS as is the case for regular Bootstrap widgets.

BUG=
R=kevmoo@j832.com

Review URL: https://chromiumcodereview.appspot.com/17965002