Fix email verification link

Fixes the email link so that it sends to the user to the correct page.

Also:

sets the default NODE_ENV environment variable to dev in the docker compose file.
use an npm script to launch the service, so that the container can operate in development and production modes depending on NODE_ENV.
changes to the endpoints so that they use the GET HTTP method, resulting in cleaner UI interface and interaction.

nestauk / dap_dv_backends