Apply rubocop Rails/FindBy rule everywhere

We didn't detect these cases because by default the rule only searches
for offenses in `app/models/`.

spec/models/debate_spec.rb

@@ -399,7 +399,7 @@ describe Debate do
   describe "custom tag counters when hiding/restoring" do
     it "decreases the tag counter when hiden, and increases it when restored" do
       debate = create(:debate, tag_list: "foo")
-      tag = Tag.where(name: "foo").first
+      tag = Tag.find_by(name: "foo")
       expect(tag.debates_count).to eq(1)
 
       debate.hide

spec/models/newsletter_spec.rb

@@ -117,8 +117,8 @@ describe Newsletter do
       expect(Activity.count).to eq(3)
 
       recipients.each do |email|
-        user = User.where(email: email).first
-        activity = Activity.where(user: user).first
+        user = User.find_by(email: email)
+        activity = Activity.find_by(user: user)
 
         expect(activity.user_id).to eq(user.id)
         expect(activity.action).to eq("email")

spec/models/proposal_spec.rb

@@ -341,7 +341,7 @@ describe Proposal do
   describe "custom tag counters when hiding/restoring" do
     it "decreases the tag counter when hiden, and increases it when restored" do
       proposal = create(:proposal, tag_list: "foo")
-      tag = Tag.where(name: "foo").first
+      tag = Tag.find_by(name: "foo")
       expect(tag.proposals_count).to eq(1)
 
       proposal.hide