Move third-party extensions from `public` schema to `extensions`

[Eric Hanson]

This seems to be an emerging trend, and a reasonable one given PostgreSQL extensions' architectural flaws. It's impossible to do the "right" thing here since every extension can only be installed once, and they have to be installed into a particular schema, so if two different projects need to coexist but look for an extension in different places, boom. Until we just stop using extensions entirely, write code that looks for them in the extensions schema.