Fix issue where CSS and Javascript files were permanently cached by Safari in development mode.
Safari seems to cache CSS and JS files forever, no matter what the Cache-Control header says. The
workaround is to use `Vary: *`.
Fix artist URLs still showing old cached site icons because the URL
didn't change when the file was updated. Use `image_pack_tag` so that
the filename includes the hash, so that the URL changes when the file
changes.
* Add README files to several directories in app/ giving a brief
overview of some parts of Danbooru's architecture.
* Add documentation for files in config/.
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.
