Make it possible to configure the issue notifier bot for a repo to avoid doing "forward backports"

(from PR)
The new implementation will avoid creating a "foward backport" if no suitable backport (or "forward backport") is found. That is, if JBS already contains a backport (or "forward backport") that can be used, then that backport (or "forward backport") will be used. This makes sense to me because in this scenario the user has (most likely) set up the issues in JBS for this to happen. But when Skara decides to create a new backport, then (if "avoidforwardports" is true in the configuration) the code will never create "forward backport", instead opting to create a backport with the primary issue's fixVersions and setting fixVersions of the primary issue to the value found in the repository's .jcheck/conf.