Add and apply Rails/DurationArithmetic rubocop rule

This rule was added in rubocop-rails 2.13.0. We were already applying it
most of the time.

spec/models/dashboard/action_spec.rb

@@ -77,7 +77,7 @@ describe Dashboard::Action do
 
     it "is active when published after day_offset" do
       action = build(:dashboard_action, required_supports: 0, day_offset: 10)
-      proposal = build(:proposal, published_at: Time.current - 10.days)
+      proposal = build(:proposal, published_at: 10.days.ago)
 
       expect(action).to be_active_for(proposal)
     end
@@ -91,7 +91,7 @@ describe Dashboard::Action do
 
     it "is not active when not enough time published" do
       action = build(:dashboard_action, required_supports: 0, day_offset: 10)
-      proposal = build(:proposal, published_at: Time.current - 9.days)
+      proposal = build(:proposal, published_at: 9.days.ago)
 
       expect(action).not_to be_active_for(proposal)
     end

spec/models/poll/poll_spec.rb

@@ -352,7 +352,7 @@ describe Poll do
 
   describe "#sort_for_list" do
     it "returns polls sorted by name ASC" do
-      starts_at = Time.current + 1.day
+      starts_at = 1.day.from_now
       poll1 = create(:poll, geozone_restricted: true, starts_at: starts_at, name: "Zzz...")
       poll2 = create(:poll, geozone_restricted: true, starts_at: starts_at, name: "Mmmm...")
       poll3 = create(:poll, geozone_restricted: true, starts_at: starts_at, name: "Aaaaah!")
@@ -361,7 +361,7 @@ describe Poll do
     end
 
     it "returns not geozone restricted polls first" do
-      starts_at = Time.current + 1.day
+      starts_at = 1.day.from_now
       poll1 = create(:poll, geozone_restricted: false, starts_at: starts_at, name: "Zzz...")
       poll2 = create(:poll, geozone_restricted: true, starts_at: starts_at, name: "Aaaaaah!")
 
@@ -382,7 +382,7 @@ describe Poll do
     end
 
     it "returns polls earlier to start first" do
-      starts_at = Time.current + 1.day
+      starts_at = 1.day.from_now
       poll1 = create(:poll, geozone_restricted: false, starts_at: starts_at - 1.hour, name: "Zzz...")
       poll2 = create(:poll, geozone_restricted: false, starts_at: starts_at, name: "Aaaaah!")
 
@@ -402,7 +402,7 @@ describe Poll do
       end
 
       it "orders by name considering fallback locales" do
-        starts_at = Time.current + 1.day
+        starts_at = 1.day.from_now
         poll1 = create(:poll, starts_at: starts_at, name: "Charlie")
         poll2 = create(:poll, starts_at: starts_at, name: "Delta")
         poll3 = I18n.with_locale(:es) do