Improve template helper functions: string/slice (#24266)

Follow #23328

The improvements:

1. The `contains` functions are covered by tests
2. The inconsistent behavior of `containGeneric` is replaced by
`StringUtils.Contains` and `SliceUtils.Contains`
3. In the future we can move more help functions into XxxUtils to
simplify the `helper.go` and reduce unnecessary global functions.

FAQ:

1. Why it's called `StringUtils.Contains` but not `strings.Contains`
like Golang?

Because our `StringUtils` is not Golang's `strings` package. There will
be our own string functions.

---------

Co-authored-by: silverwind <me@silverwind.io>

modules/templates/util_slice.go

@@ -0,0 +1,35 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package templates
+
+import (
+	"fmt"
+	"reflect"
+)
+
+type SliceUtils struct{}
+
+func NewSliceUtils() *SliceUtils {
+	return &SliceUtils{}
+}
+
+func (su *SliceUtils) Contains(s, v any) bool {
+	if s == nil {
+		return false
+	}
+	sv := reflect.ValueOf(s)
+	if sv.Kind() != reflect.Slice && sv.Kind() != reflect.Array {
+		panic(fmt.Sprintf("invalid type, expected slice or array, but got: %T", s))
+	}
+	for i := 0; i < sv.Len(); i++ {
+		it := sv.Index(i)
+		if !it.CanInterface() {
+			continue
+		}
+		if it.Interface() == v {
+			return true
+		}
+	}
+	return false
+}