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/comments/proposals_spec.rb

@@ -1,7 +1,7 @@
 require "rails_helper"
 include ActionView::Helpers::DateHelper
 
-feature "Commenting proposals" do
+describe "Commenting proposals" do
   let(:user) { create :user }
   let(:proposal) { create :proposal }
 
@@ -153,7 +153,7 @@ feature "Commenting proposals" do
     expect(page).to have_css(".comment", count: 2)
   end
 
-  feature "Not logged user" do
+  describe "Not logged user" do
     scenario "can not see comments forms" do
       create(:comment, commentable: proposal)
       visit proposal_path(proposal)
@@ -299,7 +299,7 @@ feature "Commenting proposals" do
     end
   end
 
-  feature "Moderators" do
+  describe "Moderators" do
     scenario "can create comment as a moderator", :js do
       moderator = create(:moderator)
 
@@ -355,7 +355,7 @@ feature "Commenting proposals" do
     end
   end
 
-  feature "Administrators" do
+  describe "Administrators" do
     scenario "can create comment as an administrator", :js do
       admin = create(:administrator)
 
@@ -411,9 +411,8 @@ feature "Commenting proposals" do
     end
   end
 
-  feature "Voting comments" do
-
-    background do
+  describe "Voting comments" do
+    before do
       @manuela = create(:user, verified_at: Time.current)
       @pablo = create(:user)
       @proposal = create(:proposal)