Recognize issue/MR/snippet/commit links as references.

app/models/commit.rb

app/models/commit_range.rb

app/models/concerns/referable.rb

app/models/issue.rb

app/models/merge_request.rb

app/models/snippet.rb

lib/gitlab/markdown.rb

lib/gitlab/markdown/abstract_reference_filter.rb

lib/gitlab/markdown/commit_range_reference_filter.rb

lib/gitlab/markdown/commit_reference_filter.rb

lib/gitlab/markdown/external_link_filter.rb

lib/gitlab/markdown/merge_request_reference_filter.rb

lib/gitlab/markdown/redactor_filter.rb

...lib/gitlab/markdown/commit_range_reference_filter_spec.rb

spec/lib/gitlab/markdown/commit_reference_filter_spec.rb

spec/lib/gitlab/markdown/issue_reference_filter_spec.rb

...ib/gitlab/markdown/merge_request_reference_filter_spec.rb

spec/lib/gitlab/markdown/snippet_reference_filter_spec.rb