Add ability to review changes as in github

[iddm]

It would be very nice to have review feature or at least an ability to comment changes in the pull request.

[krombel]

@vityafx Might this be a duplicate of #124?

[iddm]

Nope. The review feature is a bit more complicated than just comments. Have you ever done a review on github? :) So no, this is not a duplicate. I'll fix the title so it reflects the intentions correctly. Thanks for the noticing this issue.

[lafriks]

Depends on #2583

[jonasfranz]

Closed by #3748

[iddm]

Good work, thanks!