Block Migration mapping for widely-used contributed themes

Based on webchick's comment on #2248407-3: D6 Block Mapping, survey the top 10 or so widely used base themes and create map that can be used to map blocks in D6 regions to their appropriate D8 regions.

Currently, the migrate_drupal module maps regions in only D6 core themes to D8 core themes. It would be valuable if we could expand the map to include widely-used base themes as well.

Once #2248407: D6 Block Mapping is complete, the map will be a PHP array (what else?!) in the getCoreRegionMap method of Drupal\migrate_drupal\Plugin\migrate\Process\d6\BlockRegion. It goes something like this:

'D6_theme_machine_name' => array(
  'D8_theme_machine_name' => array(
    'D6_region_name' => 'D8_region_name', 
    ...
  'Another_D8_theme_machine_name' => array(
    'D6_region_name' => 'D8_region_name', 
    ...
  ...

Here's an actual snippet:

  protected static function getCoreRegionMap() {
    return array(
      'garland' => array(
        'bartik' => array(
          'left' => 'sidebar_first',
          'right' => 'sidebar_second',
          'content' => 'content',
          'header' => 'header',
          'footer' => 'footer',
        ),
        'seven' => array(
          'content' => 'content',
        ),
        'stark' => array(
          'left' => 'sidebar_first',
          'right' => 'sidebar_second',
          'content' => 'content',
          'header' => 'header',
          'footer' => 'footer',
        ),
      ),
...

thanks,
-mike