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

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

lib/auth/default_current_user_provider.rb

@@ -30,7 +30,7 @@ class Auth::DefaultCurrentUserProvider
 
     # bypass if we have the shared session header
     if shared_key = @env['HTTP_X_SHARED_SESSION_KEY']
-      uid = $redis.get("shared_session_key_#{shared_key}")
+      uid = Discourse.redis.get("shared_session_key_#{shared_key}")
       user = nil
       if uid
         user = User.find_by(id: uid.to_i)

lib/auth/open_id_authenticator.rb

@@ -87,7 +87,7 @@ class Auth::OpenIdAuthenticator < Auth::Authenticator
     omniauth.provider :open_id,
          setup: lambda { |env|
            strategy = env["omniauth.strategy"]
-           strategy.options[:store] = OpenID::Store::Redis.new($redis)
+           strategy.options[:store] = OpenID::Store::Redis.new(Discourse.redis)
 
            # Add CSRF protection in addition to OpenID Specification
            def strategy.query_string