Pushes the current branch (if safe) then publishes a GitHub release for the latest CRAN submission.
If you use devtools::submit_cran() to submit to CRAN, information about the
submitted state is captured in a CRAN-SUBMISSION file.
use_github_release() uses this info to populate the GitHub release notes
and, after success, deletes the file. In the absence of such a file, we
assume that current state (SHA of HEAD, package version, NEWS) is the
submitted state.