Add "campaign_type" field to "campaigns" table &model
For several upcoming features, we need to distinguish various campaign types:
- Test ("test") campaigns whose job is to do small test runs of Job Types (task_definitions)
- Data transfer campaigns ("xfer") which just move data around between other campaign stages
- Keepup ("keepup") campaigns that aren't ever really "done" because new data should keep appearing in their datasets
- Regular (NULL) campaigns which are complete when their dataset splitting runs out of data.
We should be able to track all of this with a single short char field added to campaigns.