refactor: wiki edit form (#38918)

1. the fragile `document.querySelector('.repository.wiki.new
.ui.form')!` is broken (again), rewrite to "data-global-init"
    * regression from #37571 because a new form was added
3. use "form-fetch-action" and JSON response instead of
"RenderWithErrDeprecated"

services/context/context.go

@@ -298,6 +298,7 @@ func GetFetchActionForm[T interface {
 	}
 	form := T(new(E))
 	errs := binding.Bind(ctx.Req, form)
+	errs = form.Validate(GetValidateContext(ctx.Req), errs)
 	errorMessage, fieldName, _ := middleware.BuildValidationErrorForUser(form, ctx.Locale, errs)
 	if errorMessage != "" {
 		ctx.Resp.Header().Set("Content-Type", "application/json")