Ignore gem name in path for MigrationClassName

[sunny]

Closes #656

Adds a fix to #644 when running the cop on migrations generated by Rails that contain the Rails module name, such as.active_storage.rb.

Currently:

db/migrate/20210217092104_add_service_name_to_active_storage_blobs.active_storage.rb:4:7: C: [Corrected] Rails/MigrationClassName: Replace with AddServiceNameToActiveStorageBlobs.activeStorage that matches the file name.
class AddServiceNameToActiveStorageBlobs < ActiveRecord::Migration[6.0]
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
db/migrate/20210217092105_create_active_storage_variant_records.active_storage.rb:4:7: C: [Corrected] Rails/MigrationClassName: Replace with CreateActiveStorageVariantRecords.activeStorage that matches the file name.
class CreateActiveStorageVariantRecords < ActiveRecord::Migration[6.0]
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This ends up autocorrecting to the following:

# This migration comes from active_storage (originally 20191206030411)
class CreateActiveStorageVariantRecords.activeStorage < ActiveRecord::Migration[6.0]
  # …
end

This fix ignores the suffix (here .active_storage).

---

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• 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.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
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[koic]

Thanks!