Move observing last build to controller

We observe last build on the repo in order to show the freshest build on
repo page. I moved it to router in order to keep such observers in the
same place, but this was not a wise move. To make it work properly
observer needs to be removed when moving to some other part (like
build's page). The problem is that deactivate function is not called
when we move to the other route in the same nesting. We have our own
'activate' function on repoController, which is better suited for
handling this task.