Start using template context function (#26254)

Before:

* `{{.locale.Tr ...}}`
* `{{$.locale.Tr ...}}`
* `{{$.root.locale.Tr ...}}`
* `{{template "sub" .}}`
* `{{template "sub" (dict "locale" $.locale)}}`
* `{{template "sub" (dict "root" $)}}`
* .....

With context function: only need to `{{ctx.Locale.Tr ...}}`

The "ctx" could be considered as a super-global variable for all
templates including sub-templates.


To avoid potential risks (any bug in the template context function
package), this PR only starts using "ctx" in "head.tmpl" and
"footer.tmpl" and it has a "DataRaceCheck". If there is anything wrong,
the code can be fixed or reverted easily.

modules/templates/helper.go

@@ -28,6 +28,8 @@ import (
 // NewFuncMap returns functions for injecting to templates
 func NewFuncMap() template.FuncMap {
 	return map[string]any{
+		"ctx": func() any { return nil }, // template context function
+
 		"DumpVar": dumpVar,
 
 		// -----------------------------------------------------------------