Unapprove button shouldn't come once the changes are merged (BB-8739)

Hey,

When my manager approves for changes made by developer, my web admin merge the changes to the master, but once the changes are merged. Manager can still see and click on unapprove button. Which is not a correct flow IMO. What if manager unapprove after a change is merged, when that change is not correct/ buggy, that eventually removes the approval messages from activity tab, too. Web admin's job is just to merge the changes(i.e. he is not aware of code much, manager does). Once it's approved web admin blindly merges the change. Any idea?