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_string.go

@@ -0,0 +1,20 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package templates
+
+import "strings"
+
+type StringUtils struct{}
+
+func NewStringUtils() *StringUtils {
+	return &StringUtils{}
+}
+
+func (su *StringUtils) HasPrefix(s, prefix string) bool {
+	return strings.HasPrefix(s, prefix)
+}
+
+func (su *StringUtils) Contains(s, substr string) bool {
+	return strings.Contains(s, substr)
+}