Make SubjectSet to Workflow relationship 1 -> *

Remove HABTM relationship between SubjectSet and Workflow so a workflow has
many subject sets and a subject set belongs to a single workflow. This will
support being able to set retirement rules on subject sets.

Closes #148. Closes #338
Merge pull request #354 from edpaget/has_one_subject_set

Make SubjectSet to Workflow relationship 1 -> *