Add ability to switch between Assigned and Review Requests merge request lists from the "Your work" section

Problem

As we've introduced the new "Your work" section (#384342 (closed)), the problem has been identified of being able to access the Review requests listing without the assistance of the menu in the top navbar. Because of this, the count for the Merge request item in the "Your work" section was only accounting for the items Assigned to you. This can cause confusion for users when within the "Your work" section, as the counts do not match up to what they see in the navbar.

Proposal

Modify the Merge Request navigation item to be a section similar to other instances in the left navigation (ex: Repository, Project Information, etc), containing items for "Assigned to you" and "Review requests for you".

menu	active/expanded

Note: When we move to the new sidebar with the navigation redesign, this will have to be handled differently as we will no longer have flyover menus.