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.
Rename the following post replacement attributes:
* file_size_was -> old_file_size
* file_ext_was -> old_file_ext
* image_width_was -> old_image_width
* image_height_was -> old_image_height
* md5_was -> old_md5
In Rails 6.1, having attributes named `file_size` and `file_size_was` on
the same model breaks things because it conflicts with Rails' dirty
attribute tracking.
