Refactor reference gathering to use a dedicated filter

app/helpers/gitlab_markdown_helper.rb

lib/gitlab/markdown/commit_range_reference_filter.rb

lib/gitlab/markdown/commit_reference_filter.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/redactor_filter.rb

lib/gitlab/markdown/reference_filter.rb

lib/gitlab/markdown/reference_gatherer_filter.rb

lib/gitlab/markdown/snippet_reference_filter.rb

lib/gitlab/markdown/user_reference_filter.rb

lib/gitlab/reference_extractor.rb

spec/helpers/gitlab_markdown_helper_spec.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

spec/lib/gitlab/markdown/label_reference_filter_spec.rb

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

spec/lib/gitlab/markdown/redactor_filter_spec.rb