bugs in upgrade tasks: 2707 and 2708

https://support.atlassian.com/browse/BSP-6845

Bug 1: Upgrade task 2707
Job tries to checkout not-existing Default repository.
Scenario when it can happen:

• Plan's repository set to None
• Several Jobs defining it's own repos.
• A Job not defining repo

It's quite complicated: repos from jobs are moved to plan. Code which decides if job needs checkout task checks if there is plan repository which is not 'None' - after moving a job repository there is. So it creates checkout task for the job and, as it doesn't have its own repo, it uses default. Later on the None repository is removed.

Bug 2: Upgrade 2708: Removal of None repository does not update position in PlanRepositoryLinks