Removed the unused global $_wp_column_headers from WPSEO_Bulk_Description_List_Table
Removed the unused $screen variable from WPSEO_Bulk_Description_List_Table
Removed unused global $_wp_column_headers from WPSEO_Bulk_Title_Editor_List_Table
Removed unused variable $screen from WPSEO_Bulk_Title_Editor_List_Table
Added check to see if post exists when updating SEO title/metadesc, returning error if not.
Merge Fix-settings-issues branch from jrfnl
Added check to see if Post Type is valid for the Post whose SEO title/metadesc is being edited.
Revert "Added check to see if Post Type is valid for the Post whose SEO title/metadesc is being edited."

This reverts commit 48835101ab96d0dba991896939696ab60b782f8d.
Added check to see if Post Type is valid for the Post whose SEO title/metadesc is being edited.
Added check to make sure the current user can edit the specified post
Removed todo comment for adding capacity check for bulk edit ajax methods.
Changed $_REQUEST to $_GET for getting 'post_type_filter'
Sanitized post types and post stati and used $wpdb->prepare when getting total posts in WPSEO_Bulk_Description_List_Table
Fixing issue caused by using $wpdb->prepare with two IN clauses. Can't pass two arrays in $wpdb-prepare that way
Escaped post types and post stati before adding to the total posts query in WPSEO_Bulk_Title_Editor_List_Table
Updated post status total queries to use $wpdb->prepare in WPSEO_Bulk_Description_List_Table
Updated post status total queries to use $wpdb->prepare in WPSEO_Bulk_Title_Editor_List_Table
Fixing a variable typo from $GET to $_GET
Replaced all uses of $_REQUEST with $_GET in WPSEO_Bulk_Description_List_Table
Replaced all uses of $_REQUEST with $_GET in WPSEO_Bulk_Title_Editor_List_Table