chore: move crates into crates/ dir (#1958)

I'd like to tidy up the root a little, and it's nice to have all the
rust crates in one place

crates/atuin-server-postgres/migrations/20210425153800_create_sessions.sql

@@ -0,0 +1,6 @@
+-- Add migration script here
+create table sessions (
+	id bigserial primary key,
+	user_id bigserial,
+	token varchar(128) unique not null
+);