When a GitHub app install completes, if there were no existing repositories available to reassign to the new HostingServiceAccount, the finish button on the wizard would incorrectly say "Cancel". This was happening because the wizard's default close button text is "Cancel" when the page does not specify its own. I've added button overrides for the two cases where we don't show a list of repos to reassign (either none available or the check failed). Testing Done: Verified that the close button was correctly labeled.