Set `crossorigin="anonymous"` on the <img> tag for thumbnails. This
makes it so we don't send session cookies in requests for thumbnails.
Since images are served from cdn.donmai.us, and since session cookies
are set on *.donmai.us, session cookies are sent by default with every
thumbnail request.
Not sending cookies saves up to 1kb of overhead per thumbnail request
(in reality, it's less than this because of HTTP/2 multiplexing and
header compression).
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.
Factor out FontAwesome icons into a set of helpers. This is so that it's
easier to keep track of which icons we're using and easier to change
icons globally.
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.
