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/features/moderation/proposals_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-feature "Moderate proposals" do
+describe "Moderate proposals" do
 
   scenario "Disabled with a feature flag" do
     Setting["process.proposals"] = nil
@@ -43,16 +43,16 @@ feature "Moderate proposals" do
     end
   end
 
-  feature "/moderation/ screen" do
+  describe "/moderation/ screen" do
 
-    background do
+    before do
       moderator = create(:moderator)
       login_as(moderator.user)
     end
 
-    feature "moderate in bulk" do
-      feature "When a proposal has been selected for moderation" do
-        background do
+    describe "moderate in bulk" do
+      describe "When a proposal has been selected for moderation" do
+        before do
           @proposal = create(:proposal)
           visit moderation_proposals_path
           within(".menu.simple") do