backport-resolve-issue: resolve multiple backport issues

backport-resolve-issue: resolve multiple backport issues

When multiple backports are concentrated into a single backport PR (a common
practice), the script would only resolve the first one, which was an annoying
shortcoming.

In addition to implementing the feature, take the opportunity to pay some
technical debt (put functions in alphabetical order, move code units into
dedicated functions/methods, etc.).

Signed-off-by: Nathan Cutler <ncutler@suse.com>