Draft: Migrate the database to schema v2

This is a partial rewrite of the database schema and the supporting application code. The goal is to prepare for larger future changes.

User impact

The only user-facing change of this patch is a fix for files created between the 23rd of February, 2019 (when the new API was deployed) and the 20th of April, 2019 (when a08978a9 was deployed). The migration script ensures that every key (now path) in the objects table (now object) has a leading slash. While this is technically a breaking change, I do not have a reason to believe any existing client would be affected.