Issue #2248407 by ultimike, hussainweb, benjy, jp.stacey: D6 Block Mapping.

...rate_drupal/config/install/migrate.migration.d6_block.yml

...upal/config/install/migrate.migration.d6_system_theme.yml

...rate_drupal/src/Plugin/migrate/process/d6/BlockRegion.php

...grate_drupal/src/Plugin/migrate/process/d6/BlockTheme.php

...les/migrate_drupal/src/Plugin/migrate/source/d6/Block.php

core/modules/migrate_drupal/src/Tests/Dump/Drupal6Block.php

...ules/migrate_drupal/src/Tests/Dump/Drupal6SystemTheme.php

.../modules/migrate_drupal/src/Tests/d6/MigrateBlockTest.php

...odules/migrate_drupal/src/Tests/d6/MigrateDrupal6Test.php

...es/migrate_drupal/src/Tests/d6/MigrateSystemThemeTest.php

...odules/system/tests/themes/test_theme/test_theme.info.yml