Use describe on new feature tests

The `type: :feature` is automatically detected by RSpec because these
tests are inside the `spec/features` folder. Using `feature` re-adds a
`type: :feature` to these files, which will result in a conflict when we
upgrade to Rails 5.1's system tests.

Because of this change, we also need to change `background` to `before`
or else these tests will fail.

We're also adding a rubocop rule so we dont' accidentally add these
keywords again.

.rubocop_basic.yml

@@ -14,6 +14,12 @@ AllCops:
   # to ignore them, so only the ones explicitly set in this file are enabled.
   DisabledByDefault: true
 
+Capybara/FeatureMethods:
+  Enabled: true
+  EnabledMethods:
+    - scenario
+    - xscenario
+
 Layout/IndentationConsistency:
   EnforcedStyle: rails