Support both internal and external issue trackers

app/controllers/projects/issues_controller.rb

app/helpers/issues_helper.rb

app/models/merge_request.rb

app/models/project.rb

app/models/project_services/issue_tracker_service.rb

app/models/project_services/jira_service.rb

app/services/issues/close_service.rb

app/views/layouts/nav/_new_project_sidebar.html.haml

app/views/layouts/nav/_project.html.haml

app/views/projects/merge_requests/index.html.haml

app/views/shared/_mr_head.html.haml

lib/api/entities.rb

lib/api/merge_requests.rb

lib/banzai/filter/issue_reference_filter.rb

lib/banzai/reference_parser/external_issue_parser.rb

lib/gitlab/reference_extractor.rb

lib/gitlab/slash_commands/issue_command.rb

spec/features/issuables/markdown_references_spec.rb

spec/features/projects/features_visibility_spec.rb

spec/helpers/issues_helper_spec.rb