Provide code review intercept on push

Problem to solve

This customer has a workflow based on Gerrit that allows them to do code review prior to accepting a push into the main repo. They are requesting similar capability in GitLab. https://gitlab.zendesk.com/agent/tickets/105187

Further details

The customer's description of that they are looking for:

Code Review in GitLab is initiated when there is a Merge Request between Git branches on the remote repository. It'll be nice to have Code Review triggered when code is pushed from a local repository branch to the corresponding branch(protected or otherwise) on the remote repository The idea is to not let any code that's not reviewed enter the remote repository in the first place, and prevent builds failing due to unreviewed code.

We’ll, however, want to retain the Code Review process during Merge Requests. This Feature Request will be in addition to that.