Issue #2638406 by NaheemSays, Deepak Goyal, heddn: Poll migrate support

migrations/poll_choice.yml

migrations/poll_field_instance.yml

migrations/poll_field_instance_display.yml

migrations/poll_field_instance_form_display.yml

migrations/poll_field_storage_config.yml

migrations/poll_question.yml

migrations/poll_reference.yml

migrations/poll_vote.yml

migrations/state/poll.migrate_drupal.yml

src/Plugin/migrate/destination/PollVote.php

src/Plugin/migrate/source/Poll.php

src/Plugin/migrate/source/PollChoice.php

src/Plugin/migrate/source/PollVote.php

tests/fixtures/drupal7.php

tests/src/Kernel/Plugin/migrate/source/PollChoiceTest.php

tests/src/Kernel/Plugin/migrate/source/PollTest.php

tests/src/Kernel/Plugin/migrate/source/PollVoteTest.php

tests/src/Kernel/migrate/d7/MigratePollTest.php