Replace .all.each with .find_each on admin specs

spec/features/admin/admin_notifications_spec.rb

@@ -199,7 +199,7 @@ feature "Admin Notifications" do
 
       expect(page).to have_content "Notification sent successfully"
 
-      User.all.each do |user|
+      User.find_each do |user|
         expect(user.notifications.count).to eq(1)
       end
     end

spec/features/admin/emails/newsletters_spec.rb

@@ -41,7 +41,7 @@ feature "Admin newsletter emails" do
 
       expect(page).to have_css(".newsletter", count: 3)
 
-      Newsletter.all.each do |newsletter|
+      Newsletter.find_each do |newsletter|
         segment_recipient = I18n.t("admin.segment_recipient.#{newsletter.segment_recipient}")
         within("#newsletter_#{newsletter.id}") do
           expect(page).to have_content newsletter.subject

spec/features/admin/signature_sheets_spec.rb

@@ -15,7 +15,7 @@ feature 'Signature sheets' do
 
       expect(page).to have_css(".signature_sheet", count: 3)
 
-      SignatureSheet.all.each do |signature_sheet|
+      SignatureSheet.find_each do |signature_sheet|
         expect(page).to have_content signature_sheet.name
       end
     end

spec/features/admin/stats_spec.rb

@@ -131,7 +131,7 @@ feature 'Stats' do
 
       expect(page).to have_css(".proposal_notification", count: 3)
 
-      ProposalNotification.all.each do |proposal_notification|
+      ProposalNotification.find_each do |proposal_notification|
         expect(page).to have_content proposal_notification.title
         expect(page).to have_content proposal_notification.body
       end