Initiate the loop for template-custom.php

I think `template-custom.php` lost the start of its loop when #1056 moved the loop-start from `templates/content-page.php` to `page.php`. A resulting problem is that `the_content()` would be empty when using the Custom Template.

This commit adds a loop-start to `template-custom.php`, mirroring the change #1056 made to `page.php`.