Custom regexp external issues (#17624)

* Implement custom regular expression for external issue tracking.

Signed-off-by: Alexander Beyn <malex@fatelectrons.org>

* Fix syntax/style

* Update repo.go

* Set metas['regexp']

* gofmt

* fix some tests

* fix more tests

* refactor frontend

* use LRU cache for regexp

* Update modules/markup/html_internal_test.go

Co-authored-by: Alexander Beyn <malex@fatelectrons.org>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/repo-legacy.js

@@ -462,6 +462,11 @@ export function initRepository() {
         if (typeof $(this).data('context') !== 'undefined') $($(this).data('context')).addClass('disabled');
       }
     });
+    const $trackerIssueStyleRadios = $('.js-tracker-issue-style');
+    $trackerIssueStyleRadios.on('change input', () => {
+      const checkedVal = $trackerIssueStyleRadios.filter(':checked').val();
+      $('#tracker-issue-style-regex-box').toggleClass('disabled', checkedVal !== 'regexp');
+    });
   }
 
   // Labels