Modify UI to avoid CORS issues when running production build

[btmorr]

#86 added a docker-compose yml file that successfully starts a 3-node cluster and a container for the UI. The UI is able to connect to nodes, but does not handle CORS or redirects correctly, so it can only write when actually connected to the current Leader.

When merged, this PR will:

• Modify the UI app to include a node app for serving the prod build (expressly for use in docker-compose, but will be generally useful)
• Remove the connect logic from the Admin page of the UI, and make this part of app launch configuration instead (using the same environment variable system for now that the database uses to determine configuration)
• Modify Dockerfiles and docker-compose.yml in keeping with UI changes