Use new fullscreen screenshot command

[birdsarah]

Geckodriver now supports full page screenshots (https://github.com/SeleniumHQ/selenium/pull/7182)

So https://github.com/mozilla/OpenWPM/blob/aa5e2a6fe628e67953016f79fddd6e9c9096ac7f/automation/CommandSequence.py#L106-L130 can be updated to work with moden geckodriver.

(originally added by pr https://github.com/mozilla/OpenWPM/pull/156)

[vringar]

This feature is not yet available to us, because it's currently only in the alpha branches of the 4.0 version of selenium. We are currently on version 3.141