Add milestone and label note types to cross refs

- Include new types in SystemNoteMetadata
- Add Label and Milestone reference_pattern to
Mentionable::ReferenceRegexes to be checked for cross references

app/models/concerns/mentionable/reference_regexes.rb

@@ -13,7 +13,9 @@ module Mentionable
     def self.other_patterns
       [
         Commit.reference_pattern,
-        MergeRequest.reference_pattern
+        MergeRequest.reference_pattern,
+        Label.reference_pattern,
+        Milestone.reference_pattern
       ]
     end
 

app/models/system_note_metadata.rb

@@ -10,6 +10,7 @@ class SystemNoteMetadata < ApplicationRecord
     commit cross_reference
     close duplicate
     moved merge
+    label milestone
   ].freeze
 
   ICON_TYPES = %w[

changelogs/unreleased/security-2920-fix-notes-with-label-cross-reference.yml

+---
+title: Show cross-referenced label and milestones in issues' activities only to authorized users
+merge_request:
+author:
+type: security