Identify and move 2-5 models

[trent-codecov]

Decide on the first batch of models and migrate them.

https://github.com/codecov/engineering-team/issues/1019

[michelletran-codecov]

Instead of migrating models, I think the better approach would be to migrate each task off using SQLAlchemy models. I've taken an inventory of the tasks and have created a spreadsheet and call tree for the tasks.

My feeling is that the tasks further down the chain (i.e. the leaves) are going to be easier to migrate, so we can start with those. The various Upload* tasks, as well as Notify task are probably big and risky, so those would need more care.