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.

lib/site_settings/local_process_provider.rb

@@ -4,7 +4,22 @@ class SiteSettings::LocalProcessProvider
 
   attr_accessor :current_site
 
-  Setting = Struct.new(:name, :value, :data_type) unless defined? SiteSettings::LocalProcessProvider::Setting
+  class Setting
+    attr_accessor :name, :data_type, :value
+
+    def value_changed?
+      true
+    end
+
+    def saved_change_to_value?
+      true
+    end
+
+    def initialize(name, data_type)
+      self.name = name
+      self.data_type = data_type
+    end
+  end
 
   def settings
     @settings[current_site] ||= {}
@@ -26,8 +41,14 @@ class SiteSettings::LocalProcessProvider
   def save(name, value, data_type)
     # NOTE: convert to string to simulate the conversion that is happening
     # when using DbProvider
-    value = value.to_s
-    settings[name] = Setting.new(name, value, data_type)
+    setting = settings[name]
+    if setting.blank?
+      setting = Setting.new(name, data_type)
+      settings[name] = setting
+    end
+    setting.value = value.to_s
+    DiscourseEvent.trigger(:site_setting_saved, setting)
+    setting
   end
 
   def destroy(name)