Use describe on 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.

spec/shared/features/imageable_destroy.rb

@@ -22,7 +22,7 @@ shared_examples "imageable destroy" do |imageable_factory_name,
 
   context "Destroy" do
 
-    background do
+    before do
       create(:image, imageable: imageable, user: imageable.author)
     end