Split sanitizer functions and fine-tune some tests (#31192) (#31200)

Backport #31192 by wxiaoguang

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/sanitizer_custom.go

@@ -0,0 +1,25 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markup
+
+import (
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/microcosm-cc/bluemonday"
+)
+
+func (st *Sanitizer) addSanitizerRules(policy *bluemonday.Policy, rules []setting.MarkupSanitizerRule) {
+	for _, rule := range rules {
+		if rule.AllowDataURIImages {
+			policy.AllowDataURIImages()
+		}
+		if rule.Element != "" {
+			if rule.Regexp != nil {
+				policy.AllowAttrs(rule.AllowAttr).Matching(rule.Regexp).OnElements(rule.Element)
+			} else {
+				policy.AllowAttrs(rule.AllowAttr).OnElements(rule.Element)
+			}
+		}
+	}
+}