[Schema Change]Add column rev_type_id to table flow_revision

Don't merge till new column is added in production

The purpose of this is to get rid of two-column revision tables like
flow_header_revision. This change involves multiple steps, this patch
is step 1 of the following:

1. Add the new column and start populating data on new records

2. Create a maintenance script and populate data for old records

3. Have post_id/work_flow_id reference flow_revision.rev_type_id, remove
   reference to flow_header_revision.header_workflow_id

4. Drop obsolete table: flow_header_revision

Change-Id: I9274879ac9ddd5cff9fa25bab4c1ab350c70b94d