Simplified startup in Firefly notebook

[stargaser]

With the w_2018_20 build of lsst_distrib, the display_firefly backend and firefly_client.FireflyClient can be initialized much more easily, thanks to code updates and environment variables now provided.

import lsst.afw.display as afw_display
afw_display.setDefaultBackend('firefly')
display1 = afw_display.getDisplay(frame=1)

A link will be displayed for the user to open up the Firefly viewer in another browser tab.

The underlying instance of FireflyClient can be accessed like this:

fc = display1._client

This startup procedure avoids the need to discuss servers or channels.