Create locale helper for global translations

There are some translation strings we should use across multiple views.
Instead of duplicating the translations, we can use a different locale
helper method, `#tt`, to get a hash of all available translations.

Then, in a partial view like `pagination.mustache`, we can render
translations regardless of what the current view class is.

This commit adds the necessary helper, tests, and uses the new method to
render translations on the `pagination.mustache` template, which is used
by many other view classes (latest changes, history, and search).

test/gollum/views/test_locale_helper.rb

@@ -132,4 +132,99 @@ describe Precious::Views::LocaleHelpers do
       end
     end
   end
+
+  describe "#tt" do
+    describe "mustache usage" do
+      let(:subject) { dummy_instance.render(mustache_template) }
+
+      let(:mustache_template) { "{{ tt.test_class.hello_world }}" }
+
+      describe "in the default locale" do
+        it "returns the translation string" do
+          _(subject).must_equal "Hello world"
+        end
+      end
+
+      describe "in the configured locale" do
+        it "returns the translation string" do
+          I18n.locale = :de
+
+          _(subject).must_equal "Hallo Welt"
+        end
+      end
+
+      describe "translations with YAML arguments" do
+        let(:mustache_template) { "{{ tt.test_class.author_info.full }}" }
+
+        describe "in the default locale" do
+          it "autofills YAML arguments" do
+            _(subject).must_equal "Author J.R.R. is from Bloemfontein"
+          end
+        end
+
+        describe "in the configured locale" do
+          it "autofills YAML arguments" do
+            I18n.locale = :de
+
+            _(subject).must_equal "Autor J.R.R. ist vom Bloemfontein"
+          end
+        end
+      end
+
+      describe "translations with invalid arguments" do
+        let(:mustache_template) { "{{ tt.test_class.has_invalid_argument }}" }
+
+        it "fails gracefully with embedded error message" do
+          expected_string = "Welcome to " \
+            "[#{TestClass::NO_METHOD_MESSAGE}: no_matching_method]"
+
+          _(subject).must_equal expected_string
+        end
+      end
+
+      describe "missing translations" do
+        let(:mustache_template) {
+          "{{ tt.test_class.nested.nonexistent_key }}"
+        }
+
+        it "outputs an empty string" do
+          _(subject).must_be_empty
+        end
+      end
+    end
+
+    describe "usage" do
+      let(:subject) { dummy_instance.tt }
+
+      it "returns a hash" do
+        _(subject).must_be_kind_of Hash
+      end
+
+      it "returns all present translation keys" do
+        i18n_keys = I18n.t(".").keys
+
+        _(subject.keys).must_equal i18n_keys
+      end
+
+      it "returns nested keys" do
+        nested_keys = subject[:test_class][:author_info].keys
+
+        _(nested_keys).must_equal [:full]
+      end
+
+      describe "auto-filled YAML arguments" do
+        let(:subject) { dummy_instance.tt[:test_class][:author_info][:full] }
+
+        it "auto-fills in the default locale" do
+          _(subject).must_equal "Author J.R.R. is from Bloemfontein"
+        end
+
+        it "auto-fills in a configured locale" do
+          I18n.locale = :de
+
+          _(subject).must_equal "Autor J.R.R. ist vom Bloemfontein"
+        end
+      end
+    end
+  end
 end