react-rust-postgres: add connection to postgres with migrations

Signed-off-by: Jérémie Drouet <jeremie.drouet@gmail.com>

react-rust-postgres/backend/migrations/2020-03-26-084736_create_people/down.sql

@@ -0,0 +1 @@
+DROP TABLE users;

react-rust-postgres/backend/migrations/2020-03-26-084736_create_people/up.sql

@@ -0,0 +1,4 @@
+CREATE TABLE users (
+  id SERIAL PRIMARY KEY,
+  login TEXT UNIQUE NOT NULL
+);