Improvements to firefly_client and display_firefly to allow initialization without specifying Firefly server or channel, in these two use cases:

1. User in a Jupyterlabdemo environment which also has a Firefly server
2. User running on own machine with dockerized Firefly server

• Rename the first parameter for initializing FireflyClient to be url, and remove the basedir parameter by folding it into url. An example of url is https://lsst-demo.ncsa.illinois.edu/firefly.
• When firefly_client  is imported, default url to the value of the environment variable FIREFLY_URL if it is defined. If it is not defined, default to http://localhost:8080/firefly. For the parameter html_file, default to the value of the environment variable FIREFLY_HTML if it is defined; if not defined, default to None which is equivalent to an empty string.
• By default, do not specify channel and allow it to be auto-generated. In display_firefly, expose the channel, the browser url, and the instance of FireflyClient
• In FireflyClient.launch_browser, check if the webbrowser open has failed, and display a message that the browser couldn't be auto-opened, directing the user to open the url in a browser window. In the Jupyter notebook, the message is a clickable link.
• Update documentation, and in the case of firefly_client, example notebooks.
• Reorganize display_firefly documentation for publication to pipelines.lsst.io.