Consider setting up trusat-backend with GIT sub-modules

[interplanetarychris]

To provide better configuration control across trusat-backend, trusat-frontend and trusat-orbit, we should consider using GIT's native sub-module feature .

A quick look would seem to imply that a reasonable configuration would be to have trusat-backend be the "primary" repository, with -frontend and -orbit as sub-modules (tagged to a specific version/release/branch).