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>

services/forms/repo_form.go

@@ -141,6 +141,7 @@ type RepoSettingForm struct {
 	ExternalTrackerURL                    string
 	TrackerURLFormat                      string
 	TrackerIssueStyle                     string
+	ExternalTrackerRegexpPattern          string
 	EnableCloseIssuesViaCommitInAnyBranch bool
 	EnableProjects                        bool
 	EnablePackages                        bool