Allow parent-child relationships between girder job models

[kotfic]

Deliverables:

• Add attribute to job model in girder that references parrent
• Add utilities to query job collection to get list of child tasks

Technical Notes:

• In the case of groups of tasks, how do we handle the failure of an individual task? In some cases we want the group to fail, in others we want the group to continue.
• How to visualise the parent child relationship (keeping an eye towards supporting canvas features).

Depends on: None