Revert "Revert "Merge branch 'master' of https://github.com/discourse/discourse""

This reverts commit 20780a1eee.

* SECURITY: re-adds accidentally reverted commit:
  03d26cd6: ensure embed_url contains valid http(s) uri
* when the merge commit e62a85cf was reverted, git chose the 2660c2e2 parent to land on
  instead of the 03d26cd6 parent (which contains security fixes)

config/initializers/001-redis.rb

@@ -2,5 +2,20 @@
 
 if Rails.env.development? && ENV['DISCOURSE_FLUSH_REDIS']
   puts "Flushing redis (development mode)"
-  Discourse.redis.flushall
+  Discourse.redis.flushdb
+end
+
+if ENV['RAILS_FAILOVER']
+  message_bus_keepalive_interval = MessageBus.keepalive_interval
+
+  RailsFailover::Redis.register_master_up_callback do
+    MessageBus.keepalive_interval = message_bus_keepalive_interval
+    Discourse.clear_readonly!
+    Discourse.request_refresh!
+  end
+
+  RailsFailover::Redis.register_master_down_callback do
+    # Disables MessageBus keepalive when Redis is in readonly mode
+    MessageBus.keepalive_interval = 0
+  end
 end