alvaroaleman: Checkconfig: Add a check to verify the GitHub app is installed

JobBuild
23051
07-28 19:30
pull-test-infra-bazel 1420722853308796928 1420466578859757568
pull-test-infra-integration 1420722853413654528 1420466578968809472
pull-test-infra-verify-file-perms 1420722853342351360 1420466578893312000
pull-test-infra-yamllint 1420722853380100096 1420466578926866432