Feature/2237 auto generate screenshots for user manual

Hi @aronasorman - this is for #2237... still a WIP but already working, see ToDos section below.

This PR has the `python manage.py screenshots` management command.  It needs the `/kalite/distributed/data/screenshots.json` to take screenshots using Selenium firefox webdriver.

It auto-creates another sqlite3 database on `/data/screenshots/screenshots-data.sqlite` and also saves the screenshots (.png files) on the same location.  It then deletes that sqlite3 database when done.

It inherits form the `KALiteDistributedBrowserTestCase` class for reuse.

There's a documentation at `/docs/screenshots.md`.

**ToDos**

1. Complete the `screenshots.json` based on the User Guide v0.12.x
1. Need data on what account to use for the KA-Lite Hub to simulate selenium web browser - unless it's ok to just create another user for the device registration and Hub signup.
This return result also isn't being used anywhere, maybe it's best to just delete it and have the function return `None`.