Add new Rails/InBatchesMigrationTransaction cop

[bart-westenenk-bex]

This new cop enforces the use of disable_ddl_transaction! when a migration uses in_batches to go over a lot of records.
In general, when you use in_batches within a transaction you will negate the effect of using in_batches and when deploying the migration you will put a full write lock on the full relation instead of the batched version of it.

Since you must always check yourself if it is sensible to use disable_ddl_transaction!, I have not implemented an autocorrector for this cop.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide. ==> Add Avoid in_batches without disable_ddl_transaction rails-style-guide#377