Drop the definition model

The definition hangs around but doesn't do anything for us currently. Its
an idea for the future of user defined workflows, but we don't really know
how those might work yet.  It requires a network request or two on every
page and gives us little in return. Based on this lets just remove it for
now and if needed later can be revived.

Deployment process
* DONE Add unused workflow_type field
* DONE Deploy patch prior to this one adding code to populate
  workflow_type field
* DONE Run SQL to set the workflow_type fields
* DONE Change flow_workflow.workflow_definition_id in production to allow null
* Deploy this patch that no longer needs definition table
* Drop flow_definition table and workflow_definition_id column

Change-Id: I470e7f1705166420d465df72f907c18e61ee0f89
Merge "Drop the definition model"