Refactor line check

Recreational refactoring and fighting against a **C** mark on https://codeclimate.com/github/envato/double_entry/DoubleEntry::LineCheck

Next step: Extract methods into a `LineChecker` *Service* Model, because a `LineCheck` database entry is something completely different thing than a `LineChecker` that checks all the lines.

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@spickermann you didn't align all the things!? Are you feeling ok?