This PR adds a rudimentary make_appimage script (called by the make appimage command) that uses linuxdeploy to package PyInstaller-generated binaries into a single, standalone, executable Gridsync AppImage file. Pending further testing and integration with CI/CD, this preliminarily allows for Gridsync AppImages to be offered alongside future releases, making it considerably easier for GNU/Linux users to install and run the application.

Closes #245