Improve template helper (#24417)

It seems that we really need the "context function" soon. So we should
clean up the helper functions first.

Major changes:

* Improve StringUtils and add JsonUtils
* Remove one-time-use helper functions like CompareLink
* Move other code (no change) to util_avatar/util_render/util_misc (no
need to propose changes for them)

I have tested the changed templates:


![image](https://user-images.githubusercontent.com/2114189/235283862-608dbf6b-2da3-4d06-8157-b523ca93edb4.png)


![image](https://user-images.githubusercontent.com/2114189/235283888-1dfc0471-e622-4d64-9d76-7859819580d3.png)


![image](https://user-images.githubusercontent.com/2114189/235283903-d559f14d-4abb-4a50-915f-2b9cbc381a7a.png)


![image](https://user-images.githubusercontent.com/2114189/235283955-b7b5adea-aca3-4758-b38a-3aae3f7c6048.png)

---------

Co-authored-by: Giteabot <teabot@gitea.io>

modules/templates/util_json.go

@@ -0,0 +1,35 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package templates
+
+import (
+	"bytes"
+
+	"code.gitea.io/gitea/modules/json"
+)
+
+type JsonUtils struct{} //nolint:revive
+
+var jsonUtils = JsonUtils{}
+
+func NewJsonUtils() *JsonUtils { //nolint:revive
+	return &jsonUtils
+}
+
+func (su *JsonUtils) EncodeToString(v any) string {
+	out, err := json.Marshal(v)
+	if err != nil {
+		return ""
+	}
+	return string(out)
+}
+
+func (su *JsonUtils) PrettyIndent(s string) string {
+	var out bytes.Buffer
+	err := json.Indent(&out, []byte(s), "", "  ")
+	if err != nil {
+		return ""
+	}
+	return out.String()
+}