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/web/auth/oauth.go

@@ -578,7 +578,7 @@ func GrantApplicationOAuth(ctx *context.Context) {
 
 // OIDCWellKnown generates JSON so OIDC clients know Gitea's capabilities
 func OIDCWellKnown(ctx *context.Context) {
-	t, err := ctx.Render.TemplateLookup("user/auth/oidc_wellknown")
+	t, err := ctx.Render.TemplateLookup("user/auth/oidc_wellknown", nil)
 	if err != nil {
 		ctx.ServerError("unable to find template", err)
 		return