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.

routers/install/install.go

@@ -68,9 +68,13 @@ func Contexter() func(next http.Handler) http.Handler {
 			}
 			defer baseCleanUp()
 
+			ctx.TemplateContext = context.NewTemplateContext(ctx)
+			ctx.TemplateContext["Locale"] = ctx.Locale
+
 			ctx.AppendContextValue(context.WebContextKey, ctx)
 			ctx.Data.MergeFrom(middleware.CommonTemplateContextData())
 			ctx.Data.MergeFrom(middleware.ContextData{
+				"Context":        ctx, // TODO: use "ctx" in template and remove this
 				"locale":         ctx.Locale,
 				"Title":          ctx.Locale.Tr("install.install"),
 				"PageIsInstall":  true,