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/admin_progressable.rb

@@ -1,7 +1,7 @@
 shared_examples "admin_progressable" do |factory_name, path_name|
   let!(:progressable) { create(factory_name) }
 
-  feature "Manage progress bars" do
+  describe "Manage progress bars" do
     let(:progressable_path) { send(path_name, *resource_hierarchy_for(progressable)) }
 
     let(:path) do