Since #26254, it started using `{{ctx.Locale.Tr ...}}`
Now the `ctx` seems stable enough, so the check could be removed.
(cherry picked from commit 567a68a0bf78c8d70f08c8ab948fdbb455225aa9)
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.
wxiaoguang