allow setting cookie max age

[nvdk]

In some settings 'session cookies' are not preferred, since they can be very long lived. This allows you to set the cookie max age to auto expire cookies

Can be tested using the following docker compose snippet:

  identifier:
    image: nvdk/mu-identifier:feature-cookie-max-age
    links:
      - dispatcher:dispatcher
    environment:
      SESSION_COOKIE_MAX_AGE: 30

This sets a cookie that expires after 30 seconds