Expand WorkflowConfiguration to apply to multiple WorkflowVersions

@johnbradley and I discussed on a call today.

WorkflowConfiguration was added in https://github.com/Duke-GCB/bespin-api/pull/165 and has a FK relationship to a single WorkflowVersion In practice, it will be common to add more workflow versions and want to keep compatibility with existing WorkflowConfirations.

So I propose that

- WorkflowConfigurations do not FK to a single WorkflowVersion.
- We could:
- add a many-to-many relationship,
- FK them to a Workflow (not a workflow version)
- Keep the relationship deliberately loose

if there's no explicit relationship, we could let users choose any configuration with any workflow version when creating a job. The proposed job validation step could check that the fields make sense.

I likened the WorkflowConfiguration to a JSON file that you might re-use across versions, and I think that's an easy model to explain.