Add environment variable to front-end (bot-ui) for api (bot-api)

[rwilson-release]

The front-end javascript app is hardcoded to http://localhost:8504 but this does not leave room for customisation on the port number or the host (for example, at release.com we can deploy this stack on a customer's cloud for them to test). This patch should allow you to create an environment variable at static build or runtime to configure the location of the bot's api endpoint.

Any questions or concerns, let me know. I hope this conforms to submission guidelines and tests, etc.