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>

modules/regexplru/regexplru_test.go

@@ -0,0 +1,27 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package regexplru
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRegexpLru(t *testing.T) {
+	r, err := GetCompiled("a")
+	assert.NoError(t, err)
+	assert.True(t, r.MatchString("a"))
+
+	r, err = GetCompiled("a")
+	assert.NoError(t, err)
+	assert.True(t, r.MatchString("a"))
+
+	assert.EqualValues(t, 1, lruCache.Len())
+
+	_, err = GetCompiled("(")
+	assert.Error(t, err)
+	assert.EqualValues(t, 2, lruCache.Len())
+}