Release 4.0.15

Fix up release builds and process:

Pull libbiblesync.dll from system installation location
Install biblesync from Fedora package repos
Remove libgsf, which is no longer needed
Allow building the container image to avoid re-downloading dependencies,
over and over again
Don't include new build files in package
Upload both .tar.gz and .zip files
Don't generate RPM or deb files
Simplify logic for source upload, since Github Actions will detect
duplicates, and only the first file will be accepted
v2 of {upload,download}-artifact actions now has greatly streamlined
tools so we don't have to be so uptight about how things are specified.
Move to using this
Update docs