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/proposal_notifications_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-feature "Proposal Notifications" do
+describe "Proposal Notifications" do
 
   scenario "Send a notification" do
     author = create(:user)
@@ -363,7 +363,7 @@ feature "Proposal Notifications" do
 
     context "Group notifications" do
 
-      background do
+      before do
         Setting[:proposal_notification_minimum_interval_in_days] = 0
       end