Review to Comment migration destroys newlines

![grafik](https://user-images.githubusercontent.com/1366654/36469162-9421f7ac-16e6-11e8-8b66-987801daabe1.png)

Newlines have to be kept at migration. People tend to write much text at reviews and paragraphs help structuring their thoughts.

Please update the migration code!

Refs https://github.com/JabRef/jabref/pull/3658

