Move to a new database schema diff library

[davenquinn]

migra has a fantastic usage pattern but is unmaintained. This has caused some issues like #37 but these will only grow.

The capability to "diff" Postgres schemas is invaluable for migrating Macrostrat's database over time without a lot of overhead. We're dependent on it, in particular, for automated tests of our migrations suite. Other libraries exist for this function; eventually we will want to drop one in. Ideally, the module would be in Python for easy integration, but this is negotiable if a full-featured solution is found – we'll mostly be running this in Docker contexts anyway.

We'll collect projects of interest here:

• https://github.com/stripe/pg-schema-diff
• https://squawkhq.com/

[davenquinn]

@amyfromandi FYI – this is a small but important element of our database migration approach

[davenquinn]

See djrobstep/results#7 for more recent context. results seems to be a good option at the moment, but it too has an uncertain future.