DEV: s/\$redis/Discourse\.redis (#8431)

This commit also adds a rubocop rule to prevent global variables.

spec/requests/webhooks_controller_spec.rb

@@ -3,7 +3,7 @@
 require "rails_helper"
 
 describe WebhooksController do
-  before { $redis.flushall }
+  before { Discourse.redis.flushall }
 
   let(:email) { "em@il.com" }
   let(:message_id) { "12345@il.com" }