When skipping checks due to a label, update existing statuses if present

[astrofrog]

When skipping checks, we currently post to the single status with the bot name (no e.g. :changelog):

            if skip_fails:
                pr_handler.set_status('failure', "Skipping checks due to {0} label".format(label), current_app.bot_username)

However, if post_comment is False and there are already individual status checks (e.g. astropy-bot:changelog) we should mark those as skipped instead of creating a astropy-bot status.

[pllim]

Where is this code? I failed to "grep" it from master.