Add an ApiToken model, and an endpoint to get one

migrations/postgres/2018-10-21-163227_create_api_token/down.sql

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+-- This file should undo anything in `up.sql`
+DROP TABLE api_tokens;

migrations/postgres/2018-10-21-163227_create_api_token/up.sql

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+-- Your SQL goes here
+CREATE TABLE api_tokens (
+    id SERIAL PRIMARY KEY,
+    creation_date TIMESTAMP NOT NULL DEFAULT now(),
+    value TEXT NOT NULL,
+    scopes TEXT NOT NULL,
+    app_id INTEGER NOT NULL REFERENCES apps(id) ON DELETE CASCADE,
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE
+)