Watch for PRs for the Sandbox team, instead of Owners.

Previously, PRs were only being watched for by members of the Owners team. As part of improving on-boarding, this PR takes advantage of @antoviaque's addition of the "Sandbox" team, by watching for PRs against edx/edx-platform by members of that group instead. This is to make it so that candidates can also automatically have sandboxes provisioned for them, but also to make it easier for us to off-board.