Add Media Library context/step definitions

[paulgibbs]

Expected behaviour

If you're suggesting a change/improvement, tell us how it should work.

Current behaviour

If suggesting a change/improvement, explain the difference from current behaviour.

Context

How has this issue affected you? What are you trying to accomplish? Providing context helps us come up with a solution that is most useful in the real world.

Your environment

Include as many relevant details about the environment you experienced the bug in:

• WordHat version used:
• Environment name and version (e.g. PHP 7.1 on nginx 1.11.9):
• Server operating system type and version:
• Type of Behat browser used (e.g. Selenium, headless Chrome, etc):
• Link to your project:

[paulgibbs]

Include support for media sideloading.