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/proposals_spec.rb

@@ -1,7 +1,7 @@
 # coding: utf-8
 require "rails_helper"
 
-feature "Proposals" do
+describe "Proposals" do
 
   it_behaves_like "milestoneable",
                   :proposal,
@@ -646,7 +646,7 @@ feature "Proposals" do
     expect(page).to have_content error_message
   end
 
-  feature "Proposal index order filters" do
+  describe "Proposal index order filters" do
 
     scenario "Default order is hot_score", :js do
       create_featured_proposals
@@ -831,7 +831,7 @@ feature "Proposals" do
     end
   end
 
-  feature "Archived proposals" do
+  describe "Archived proposals" do
 
     scenario "show on archived tab" do
       create_featured_proposals
@@ -1556,7 +1556,7 @@ feature "Proposals" do
 
     context "By geozone" do
 
-      background do
+      before do
         @california = Geozone.create(name: "California")
         @new_york   = Geozone.create(name: "New York")
 
@@ -1745,7 +1745,7 @@ feature "Proposals" do
 
 end
 
-feature "Successful proposals" do
+describe "Successful proposals" do
 
   scenario "Successful proposals do not show support buttons in index" do
     successful_proposals = create_successful_proposals