DEV: Replace site_setting_saved DiscourseEvent with site_setting_changed (#7401)

* DEV: Replace site_setting_saved DiscourseEvent with site_setting_changed

site_setting_saved is confusing for a few reasons:
- It is attached to the after_save of the ActiveRecord model. This is confusing because it only works 'properly' with the db_provider
- It passes the activerecord model as a parameter, which is confusing because you get access to the 'database' version of the setting, rather than the ruby setting. For example, booleans appear as 'y' or 'n' strings.
- When the event is called, the local process cache has not yet been updated. So if you call SiteSetting.setting_name inside the event handler, you will receive the old site setting value

I have deprecated that event, and added a new site_setting_changed event. It passes three parameters:
- Setting name (symbol)
- Old value (in ruby format)
- New value (in ruby format)

It is triggered after the setting has been persisted, and the local process cache has been updated.

This commit also includes a test case which describes the confusing behavior. This can be removed once site_setting_saved is removed.

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

@@ -1,19 +1,15 @@
-# 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.saved_change_to_value?
-
-  if name == :must_approve_users && site_setting.value == 't'
+DiscourseEvent.on(:site_setting_changed) do |name, old_value, new_value|
+  # Enabling `must_approve_users` on an existing site is odd, so we assume that the
+  # existing users are approved.
+  if name == :must_approve_users && new_value == true
     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}/"
+    before = "/images/emoji/#{old_value}/"
+    after = "/images/emoji/#{new_value}/"
 
     Scheduler::Defer.later("Fix Emoji Links") do
       DB.exec("UPDATE posts SET cooked = REPLACE(cooked, :before, :after) WHERE cooked LIKE :like",