Add database username and password to .env file

If one does not use the default postgress username and password in their local postgres installation, it would be nice if they are able to change the database username and password in a .env file which is not tracked by git. For a person using the default username and password, the workflow should remain as is currently.

## Expected Output
Developers can change their database credentials in the .env file

## Possible Solution
Update the `database.yml` to use environmental variables