Restart individual matrix jobs instead of all jobs

[bkimminich]

Clicking the restart button in the app triggers a "Restart Build" independent of whether you are currently viewing the whole build or one specific build matrix job. When in the view for the latter, the button should trigger "Restart Build" instead, similar to the Travis-CI web UI:

Travis-CI shows texts accordingly, in the app you could use slightly different button images to make this distinction obvious to the user.