Move default connection generation in to server endpoint

Currently, the default connection is configured in a myriad of ways.

- Environment values set at docker run time
- Environment values set in `.env` file
- A `config.json` file that sets environment values

Then a Bash script interprets all of these options and generates a `defaultConnection.json` file. That file is then served as a static file by the `/defaultConnection` endpoint in Express.

Instead, I propose simplifying the approach a bit. The `/defaultConnection` endpoint can read the environment values, validate them, and generate the proper JSON object to send to the client. This removes complex and brittle Bash code to parse and manipulate JSON files and provides an easier path to testing the logic. It also allows for better defaults and fallback values.

## Related issues

- Similar idea #1265