posts: rework post events page.

* 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).

app/logical/source/url.rb

@@ -185,6 +185,10 @@ module Source
       nil
     end
 
+    def self.site_name(url)
+      Source::URL.parse(url)&.site_name
+    end
+
     def self.image_url?(url)
       Source::URL.parse(url)&.image_url?
     end