Good article about the difference and necessity of differentiation between architecture and implementation code review. With a nice procedure how to do.
My preferred tool for reviewing my source code on github is gh-review.
In my current development work I had to check commits, packages/modules. This leads to the differentiation between commit review and technical reviews. We used commit reviews to find any sort of bug but it is hard to find design problems in inspecting only a commit without knowing the rest of the code so introduced technical reviews that watched whole classes and packages. We do commit reviews daily and technical reviews after introducing a feature and by request. The following tools help us by doing these types of reviewing.
Sonarqube is full featured code analyzing tool and one part of it is allowing to comment on your code. With this feature you can do technical reviews.
Barkeep is review tool specialized on commit reviews. It does this and does it very well.