Add `reference_pattern` to Referable models

app/models/commit.rb

app/models/commit_range.rb

app/models/concerns/referable.rb

app/models/external_issue.rb

app/models/group.rb

app/models/issue.rb

app/models/label.rb

app/models/merge_request.rb

app/models/project.rb

app/models/snippet.rb

app/models/user.rb

lib/gitlab/markdown/commit_range_reference_filter.rb

lib/gitlab/markdown/commit_reference_filter.rb

lib/gitlab/markdown/cross_project_reference.rb

lib/gitlab/markdown/external_issue_reference_filter.rb

lib/gitlab/markdown/issue_reference_filter.rb

lib/gitlab/markdown/label_reference_filter.rb

lib/gitlab/markdown/merge_request_reference_filter.rb

lib/gitlab/markdown/snippet_reference_filter.rb

lib/gitlab/markdown/user_reference_filter.rb