Add doesCustomerExistInDB function

Relates #25 #37 #60

src/lib/doesCustomerExistInDB.js

@@ -0,0 +1,20 @@
+const connect = require('../../database/db_connection.js');
+
+const doesCustomerExistInDB = (userCredentials, callback) => {
+  // query the database
+  const databaseQuery = 'SELECT * FROM users WHERE stripe_id IS NOT NULL AND github_username = $1';
+  const databaseParams = [userCredentials.username];
+  connect.query(databaseQuery, databaseParams, (err, result) => {
+    if (err) {
+      callback(err);
+    } else if (result.rows.length === 0) {
+      callback(null, false);
+    } else {
+      callback(null, result.rows[0]);
+    }
+  });
+  // if user has stripe id then return it
+  // otherwise return false
+};
+
+module.exports = doesCustomerExistInDB;