* Add a global /post_events page that shows the history of all approvals,
disapprovals, flags, appeals, and replacements on a single page.
* Redesign the /posts/:id/events page to show all approval, disapproval,
flag, appeal, and replacement events for a single post (before it only
showed approvals, flags, and appeals).
* Remove the replacement history link from the post show page. Replacements
are now included in the post events page (closes#4948: Highlighed replacements).
* Add /post_approvals/:id and /post_replacements/:id routes (these are
used by the "Details" link on the post events page).
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.
