Switch back to git submodules and pack source release
Created by: gasinvein
Reverts commit eabf0482 Add circle-flags and fluent-icon-theme submodules not present in the original git source tree with submodules. Set submodule tags to match external project tags set as of commit b0ed0757
Add GH action for packing full source tarball and adding it to release assets (addresses #116 (closed)).