Sprint automation: when a PR links to an issue, move the issue across the sprint board with the PR

[mbestavros]

The Enarx sprint framework treats pull requests differently from issues: they have their own workflow for being reviewed and merged.

Pull requests frequently link to issues in order to automatically close them. However, this means that when a PR is added to the sprint board in order to track its progress, the task encapsulated by its linked issue is duplicated (both the issue and the linked PR are present, when they refer to the same task).

To minimize this duplication, we should have issues move alongside PRs when they are linked.