Migrate existing database schema to new schema

[jmgasper]

We've changed the schema recently:

#43
#77

We've added owner and copilot files, and deprecated the username field.

The problem is that we have existing prod data with the old schema and we can't lose those old projects.

We need to ensure that projects on the old schema are properly updated to the new schema. For now, the owner and copilot fields can be set to the value of the username field.

[jmgasper]

This seems to also affect ownership transfer of an old challenge without the new schema: