Normalize workflow step parameters earlier...

... this will allow linking to actual steps (referential integrity) if/when we store workflow requests in the database.

Also unit tests.
Normalize workflow input dataset dict earlier...

... this will allow linking to actual steps (referential integrity) if/when we store workflow requests in the database and failing faster.

Also more unit tests.
weblessgx: Separate out app-support mixin for reuse from GalaxyWebTransaction...

... and unit test.
Add WorkflowRequest and associated infrastructure to data model.

This is a stripped down version of data model changes made by Kyle Ellrott for work on Galaxy Farm https://bitbucket.org/kellrott/galaxy-farm/branch/workflow_migrate.
WIP: Implement work request and workflow request infrastructure.