Apply Style/StringLiteralsInInterpolation rubocop rule

We were already using it in most places.

spec/features/admin/poll/booth_assigments_spec.rb

@@ -194,13 +194,13 @@ describe "Admin booths assignments" do
       end
 
       within("#recounts_list") do
-        within("#recounting_#{poll.starts_at.to_date.strftime('%Y%m%d')}") do
+        within("#recounting_#{poll.starts_at.to_date.strftime("%Y%m%d")}") do
           expect(page).to have_content 1
         end
-        within("#recounting_#{(poll.ends_at.to_date - 5.days).strftime('%Y%m%d')}") do
+        within("#recounting_#{(poll.ends_at.to_date - 5.days).strftime("%Y%m%d")}") do
           expect(page).to have_content "-"
         end
-        within("#recounting_#{poll.ends_at.to_date.strftime('%Y%m%d')}") do
+        within("#recounting_#{poll.ends_at.to_date.strftime("%Y%m%d")}") do
           expect(page).to have_content 1
         end
       end

spec/mailers/dashboard/mailer_spec.rb

@@ -197,9 +197,9 @@ describe Dashboard::Mailer do
                                       "among other features and rewards that you will discover. "\
                                       "Dont stop adding support and we will not stop rewarding "\
                                       "and helping you!")
-      expect(email).to have_body_text("You have #{Setting['months_to_archive_proposals']} months "\
+      expect(email).to have_body_text("You have #{Setting["months_to_archive_proposals"]} months "\
                                       "since you publish the proposal to get "\
-                                      "#{Setting['votes_for_proposal_success']} support and your "\
+                                      "#{Setting["votes_for_proposal_success"]} support and your "\
                                       "proposal can become a reality. But the first days are the "\
                                       "most important. It is a challenge. Get ready!")
       expect(email).to have_body_text("And for you to start with all the motivation,")

spec/models/proposal_spec.rb

@@ -128,7 +128,7 @@ describe Proposal do
   it "has a code" do
     Setting["proposal_code_prefix"] = "TEST"
     proposal = create(:proposal)
-    expect(proposal.code).to eq "TEST-#{proposal.created_at.strftime('%Y-%m')}-#{proposal.id}"
+    expect(proposal.code).to eq "TEST-#{proposal.created_at.strftime("%Y-%m")}-#{proposal.id}"
 
     Setting["proposal_code_prefix"] = "MAD"
   end