FEATURE: Update upload security status on post move, topic conversion, category change (#8731)

Add TopicUploadSecurityManager to handle post moves. When a post moves around or a topic changes between categories and public/private message status the uploads connected to posts in the topic need to have their secure status updated, depending on the security context the topic now lives in.

app/models/upload.rb

@@ -235,8 +235,11 @@ class Upload < ActiveRecord::Base
     return false if self.for_theme || self.for_site_setting
     mark_secure = secure_override_value.nil? ? UploadSecurity.new(self).should_be_secure? : secure_override_value
 
+    secure_status_did_change = self.secure? != mark_secure
     self.update_column("secure", mark_secure)
     Discourse.store.update_upload_ACL(self) if Discourse.store.external?
+
+    secure_status_did_change
   end
 
   def self.migrate_to_new_scheme(limit: nil)