dynamically locate sidebar template

This patch allows for `templates/sidebar-*.php` files...

For any existing template file such as 

`page.php`,  `front-page.php`, `category.php`, etc... 

a corresponding sidebar file such as 

`templates/sidebar-page.php` or `templates/sidebar-category.php` 

will be included from `base.php` using the new roots_sidebar_path().