Render milestone links as references

app/models/milestone.rb

lib/banzai/filter/abstract_reference_filter.rb

lib/banzai/filter/milestone_reference_filter.rb

lib/banzai/pipeline/gfm_pipeline.rb

lib/gitlab/reference_extractor.rb

spec/features/markdown_spec.rb

spec/fixtures/markdown.md.erb

spec/lib/banzai/filter/milestone_reference_filter_spec.rb

spec/support/markdown_feature.rb

spec/support/matchers/markdown_matchers.rb