Link to issue in CodeReuse/ActiveRecord message

This provides the user with some context as to why certain ActiveRecord
methods are blacklisted.

rubocop/cop/code_reuse/active_record.rb

@@ -9,7 +9,8 @@ module RuboCop
       class ActiveRecord < RuboCop::Cop::Cop
         include CodeReuseHelpers
 
-        MSG = 'This method can only be used inside an ActiveRecord model'
+        MSG = 'This method can only be used inside an ActiveRecord model: ' \
+        'https://gitlab.com/gitlab-org/gitlab-ce/issues/49653'
 
         # Various methods from ActiveRecord::Querying that are blacklisted. We
         # exclude some generic ones such as `any?` and `first`, as these may

spec/rubocop/cop/code_reuse/active_record_spec.rb

@@ -14,7 +14,7 @@ describe RuboCop::Cop::CodeReuse::ActiveRecord do
     expect_offense(<<~SOURCE)
     def foo
       User.where
-           ^^^^^ This method can only be used inside an ActiveRecord model
+           ^^^^^ This method can only be used inside an ActiveRecord model: https://gitlab.com/gitlab-org/gitlab-ce/issues/49653
     end
     SOURCE
   end
@@ -23,7 +23,7 @@ describe RuboCop::Cop::CodeReuse::ActiveRecord do
     expect_offense(<<~SOURCE)
     def foo
       User.where(id: 10)
-           ^^^^^ This method can only be used inside an ActiveRecord model
+           ^^^^^ This method can only be used inside an ActiveRecord model: https://gitlab.com/gitlab-org/gitlab-ce/issues/49653
     end
     SOURCE
   end
@@ -40,7 +40,7 @@ describe RuboCop::Cop::CodeReuse::ActiveRecord do
     expect_offense(<<~SOURCE)
     def foo
       project.group(:name)
-              ^^^^^ This method can only be used inside an ActiveRecord model
+              ^^^^^ This method can only be used inside an ActiveRecord model: https://gitlab.com/gitlab-org/gitlab-ce/issues/49653
     end
     SOURCE
   end