Enable featured proposals when they're used

These specs were added before we disabled featured proposals by default.
After that, they were passing, but they were not testing the scenario
they were supposed to test.

spec/features/proposals_spec.rb

@@ -1506,6 +1506,7 @@ describe "Proposals" do
     end
 
     scenario "After a search do not show featured proposals" do
+      Setting["feature.featured_proposals"] = true
       featured_proposals = create_featured_proposals
       proposal = create(:proposal, title: "Abcdefghi")
 
@@ -1644,6 +1645,7 @@ describe "Proposals" do
                   {}
 
   scenario "Erased author" do
+    Setting["feature.featured_proposals"] = true
     user = create(:user)
     proposal = create(:proposal, author: user)
     user.erase