Migrate data for issuing_org_unit.

Issuing org-unit is always taken from the first
task in a task-chain.
Merge pull request #282 from 4teamwork/deif-update-globalindex-migration

Update globalindex migration
Fix opengever task tests.
Refactor, add actor (currently for taks issuer)
Extract url-generation to actor classes.
Add unit-test for label generation and refactor label generation.
Remove dead code & Pep8.