FIX: Better handling for toggling must_approve_users

If you turn it on now, default all users to approved since they were
previously. Also support approving a user that doesn't have a reviewable
record (it will be created first.)

This also includes a refactor to move class method calls to
`DiscourseEvent` into an initializer. Otherwise the load order of
classes makes a difference in the test environment and some settings
might be triggered and others not, randomly.

config/initializers/014-track-setting-changes.rb

@@ -0,0 +1,38 @@
+# Enabling `must_approve_users` on an existing site is odd, so we assume that the
+# existing users are approved.
+DiscourseEvent.on(:site_setting_saved) do |site_setting|
+  name = site_setting.name.to_sym
+  next unless site_setting.value_changed?
+
+  if name == :must_approve_users && site_setting.value == 't'
+    User.where(approved: false).update_all(approved: true)
+  end
+
+  if name == :emoji_set
+    Emoji.clear_cache
+
+    previous_value = site_setting.attribute_in_database(:value) || SiteSetting.defaults[:emoji_set]
+    before = "/images/emoji/#{previous_value}/"
+    after = "/images/emoji/#{site_setting.value}/"
+
+    Scheduler::Defer.later("Fix Emoji Links") do
+      DB.exec("UPDATE posts SET cooked = REPLACE(cooked, :before, :after) WHERE cooked LIKE :like",
+        before: before,
+        after: after,
+        like: "%#{before}%"
+      )
+    end
+  end
+
+  Report.clear_cache(:storage_stats) if [:backup_location, :s3_backup_bucket].include?(name)
+
+  if name == :slug_generation_method
+    Scheduler::Defer.later("Null topic slug") do
+      Topic.update_all(slug: nil)
+    end
+  end
+
+  Jobs.enqueue(:update_s3_inventory) if [:s3_inventory, :s3_upload_bucket].include?(name)
+
+  SvgSprite.expire_cache if name.to_s.include?("_icon")
+end