Add and apply Rails/I18nLocaleAssignment rule

This rule was added in rubocop-rails 2.11.0.

Although we prevent I18n locale leaking between tests by setting it
before each test, the `with_locale` method makes the scope of the locale
change more obvious.

.rubocop.yml

@@ -248,6 +248,11 @@ Rails/HasManyOrHasOneDependent:
 Rails/HttpStatus:
   Enabled: true
 
+Rails/I18nLocaleAssignment:
+  Enabled: true
+  Exclude:
+    - "spec/spec_helper.rb"
+
 Rails/InverseOf:
   Enabled: true
   Exclude:

spec/i18n_spec.rb

@@ -42,13 +42,14 @@ describe "I18n" do
       I18n.backend.store_translations(:zz, {})
 
       I18n.enforce_available_locales = false
-      I18n.locale = :zz
+      I18n.with_locale(:zz) do
         I18n.fallbacks[:zz] << I18n.default_locale
 
         expect(I18n.t("test_plural", count: 0)).to eq("No comments")
         expect(I18n.t("test_plural", count: 1)).to eq("1 comment")
         expect(I18n.t("test_plural", count: 2)).to eq("2 comments")
       end
+    end
 
     it "returns count if specific plural rule not present" do
       keys = { zero:  "No comments" }

spec/models/budget_spec.rb

@@ -336,32 +336,36 @@ describe Budget do
   describe "#formatted_amount" do
     it "correctly formats Euros with Spanish" do
       budget.update!(currency_symbol: "€")
-      I18n.locale = :es
 
+      I18n.with_locale(:es) do
         expect(budget.formatted_amount(1000.00)).to eq "1.000 €"
       end
+    end
 
     it "correctly formats Dollars with Spanish" do
       budget.update!(currency_symbol: "$")
-      I18n.locale = :es
 
+      I18n.with_locale(:es) do
         expect(budget.formatted_amount(1000.00)).to eq "1.000 $"
       end
+    end
 
     it "correctly formats Dollars with English" do
       budget.update!(currency_symbol: "$")
-      I18n.locale = :en
 
+      I18n.with_locale(:en) do
         expect(budget.formatted_amount(1000.00)).to eq "$1,000"
       end
+    end
 
     it "correctly formats Euros with English" do
       budget.update!(currency_symbol: "€")
-      I18n.locale = :en
 
+      I18n.with_locale(:en) do
         expect(budget.formatted_amount(1000.00)).to eq "€1,000"
       end
     end
+  end
 
   describe "#investments_milestone_tags" do
     let(:investment1) { build(:budget_investment, :winner) }

spec/models/poll/question_spec.rb

@@ -33,10 +33,11 @@ RSpec.describe Poll::Question, type: :model do
     end
 
     context "locale with non-underscored name" do
-      before { I18n.locale = :"pt-BR" }
-
       it "correctly creates a translation" do
+        I18n.with_locale(:"pt-BR") do
           poll_question.copy_attributes_from_proposal(proposal)
+        end
+
         translation = poll_question.translations.first
 
         expect(poll_question.title).to eq(proposal.title)