Fundraising tech/Contribution tracking

Contribution tracking, as of early 2018, is the last piece of our payments infrastructure that ties the payments-wiki frontend to the database server in the backend. We want to be able to take the backend db server down for maintenance without needing to shut down the payments frontend. To do this, we will have to stop creating contribution_tracking rows directly from the wikis and move to a queue-based insert system.

Ideally we will still be able to create the ct_id on the front end, possibly using a sequence generated by redis.

Cut-off dates: TODO: till when can we safely keep working on this?