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>

templates/repo/issue/view_content/attachments.tmpl

@@ -8,7 +8,7 @@
 			<div class="gt-f1 gt-p-3">
 				<a target="_blank" rel="noopener noreferrer" href="{{.DownloadURL}}" title='{{$.ctxData.locale.Tr "repo.issues.attachment.open_tab" .Name}}'>
 					{{if FilenameIsImage .Name}}
-						{{if not (containGeneric $.Content .UUID)}}
+						{{if not (StringUtils.Contains $.Content .UUID)}}
 							{{$hasThumbnails = true}}
 						{{end}}
 						{{svg "octicon-file"}}
@@ -29,7 +29,7 @@
 		<div class="ui small thumbnails">
 			{{- range .Attachments -}}
 				{{if FilenameIsImage .Name}}
-					{{if not (containGeneric $.Content .UUID)}}
+					{{if not (StringUtils.Contains $.Content .UUID)}}
 					<a target="_blank" rel="noopener noreferrer" href="{{.DownloadURL}}">
 						<img alt="{{.Name}}" src="{{.DownloadURL}}" title='{{$.ctxData.locale.Tr "repo.issues.attachment.open_tab" .Name}}'>
 					</a>