Use globally shared HTMLRender (#24436)

The old `HTMLRender` is not ideal.

1. It shouldn't be initialized multiple times, it consumes a lot of
memory and is slow.
2. It shouldn't depend on short-lived requests, the `WatchLocalChanges`
needs a long-running context.
3. It doesn't make sense to use FuncsMap slice.


HTMLRender was designed to only work for GItea's specialized 400+
templates, so it's good to make it a global shared instance.

modules/templates/mailer.go

@@ -6,6 +6,7 @@ package templates
 import (
 	"context"
 	"html/template"
+	"regexp"
 	"strings"
 	texttmpl "text/template"
 
@@ -14,6 +15,8 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 )
 
+var mailSubjectSplit = regexp.MustCompile(`(?m)^-{3,}\s*$`)
+
 // mailSubjectTextFuncMap returns functions for injecting to text templates, it's only used for mail subject
 func mailSubjectTextFuncMap() texttmpl.FuncMap {
 	return texttmpl.FuncMap{
@@ -55,9 +58,7 @@ func Mailer(ctx context.Context) (*texttmpl.Template, *template.Template) {
 	bodyTemplates := template.New("")
 
 	subjectTemplates.Funcs(mailSubjectTextFuncMap())
-	for _, funcs := range NewFuncMap() {
-		bodyTemplates.Funcs(funcs)
-	}
+	bodyTemplates.Funcs(NewFuncMap())
 
 	assetFS := AssetFS()
 	refreshTemplates := func() {