This PR adds and makes use of a couple of basic wrappers/scripts around the gpg and signtool utilities, intended to facilitate the automation of signing- and release-related tasks -- especially on "rebranded" builds of the Gridsync application. More specifically, these scripts support reading additional signing-related parameters from config.txt, allowing, e.g., signing keys/identities to be declared in a single location. This allows individuals who wish to build and sign the application under a different identity to do so more easily -- namely, by updating the relevant values in gridsync/resources/config.txt (instead of hunting for and changing the previously-hardcoded values in the Makefile, batch scripts, etc.).