Added private method WPSEO_Metabox::get_metabox_post()

The post is fetched by checking for $_GET['post_id'] or using the global $post as a fallback. This was repeated multiple times in the class, so a single method makes sense.
Pass post ID to all WPSEO_Meta::get_value calls in WPSEO_Metabox. Fixes #1519
Merge pull request #1520 from mgmartel/metabox-post-id

Pass $post_id in all WPSEO_Meta::get_value calls in metabox - fixes #1519, props @mgmartel