Group template helper functions, remove Printf, improve template error messages (#23982)

Follow #23328 


Major changes:

* Group the function in `templates/help.go` by their purposes. It could
make future work easier.
* Remove the `Printf` helper function, there is already a builtin
`printf`.
* Remove `DiffStatsWidth`, replace with `Eval` in template
* Rename the `NewTextFuncMap` to `mailSubjectTextFuncMap`, it's for
subject text template only, no need to make it support HTML functions.


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And fine tune template error messages, to make it more friendly to
developers and users.


![image](https://user-images.githubusercontent.com/2114189/230714245-4fd202d1-2b25-41b2-8be5-03c5fee45091.png)


![image](https://user-images.githubusercontent.com/2114189/230714277-66783577-2a03-49d5-8e8c-ceba5e07a2d4.png)

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

templates/repo/issue/view_content/reference_issue_dialog.tmpl

@@ -3,7 +3,7 @@
 		{{.locale.Tr "repo.issues.context.reference_issue"}}
 	</div>
 	<div class="content" style="text-align:left">
-		<form class="ui form" action="{{Printf "%s/issues/new" .Repository.Link}}" method="post">
+		<form class="ui form" action="{{printf "%s/issues/new" .Repository.Link}}" method="post">
 			{{.CsrfTokenHtml}}
 			<div class="ui segment content">
 				<div class="field">