Share Behat feature files between Drupal 7 and 8

Ideally, we'd want to share all our Behat feature files between Panopoly 1.x for Drupal 7 and Panopoly 2.x for Drupal 8.

Here's the proposed plan:

1. #2495739: Evaluate custom Behat steps and move them upstream: Refactoring as much code out of panopoly_test/tests/steps as possible (in Drupal 7 mainly, but also 8), so that there's only minimal code there. Those files can't ever contain both D7 and D8 code because *.behat.inc files are always loaded.
2. #2854260: Port any Behat tests from Panopoly 1 that still apply to Panopoly 2
3. Tag any tests that are specific to Panopoly 1.x with @panopoly1 or Panopoly 2.x with @panopoly2 and assume any untagged tess will work with both. At the moment, we assume everything is Panopoly 1.x and only run tests tagged with panopoly2 in Drupal 8, but that's not future-focused
4. Create custom steps that abstract away the specifics that diverge between Drupal 7 and Drupal 8. At best, these should be high-level enough to make sense for both, but if not, we should base the steps on the Drupal 8 model because that is the future
5. Once the pool of features can be completely shared, we should always synchronize any changes to both versions of the panopoly_test module