Maybe you should look at RhodeCode, it has several different workflows for code-review. From very simple like just voting on each individual commit, to a fully blown server side mergable workflow that includes, voting, checks by CI, and integration to external services. We did a blog about it recently on how we use it internally: https://rhodecode.com/blog/increased-automation-at-rhodecode...

Displaimer: i'm the CTO of RhodeCode