"Given I am Logged in" support files

I had to copy these files to support tests that login first.
In browsertests, Given(/^I am logged in$/) is defined in
upload_wizard_steps.rb, so I separated this out into common_steps.rb
similar to VisualEditor's ve-mw setup.
The other definitions are straight copies from browser_tests.

Shouldn't these bits be in selenium-mediawiki ?

Change-Id: I97a54f7c7aec9d9d12455d8098673843f57d7911