Instead of changing the background color of the entire page when you
change modes in the post mode menu (which was ugly, and not obvious what
the colors meant), show a notice and make it so that hovering a post
gives it a blue outline. This indicates that clicking the post will do
something different.
* On the post show page, in the parent/child box, change it so that
currently selected post is more visible. Use a light blue outline
around the current post instead of a dark semi-transparent border.
* Fix it so that there's less empty space around thumbnails in the
parent/child box.
Refactor the post preview html to use the ViewComponent framework. This
lets us encapsulate all the HTML, CSS, and helper methods for a UI
component in a single place.
See https://viewcomponent.org.
