Use the dir attribute in dashboard and mailer layouts

We forgot to do so in commit d827768c0. In order to avoid the same
mistake in the future, we're extracting a method to get these
attributes. We're also adding tests, since we didn't have any tests to
check that the `dir` attribute was properly set.

spec/components/layout/common_html_attributes_component_spec.rb

@@ -0,0 +1,34 @@
+require "rails_helper"
+
+describe Layout::CommonHTMLAttributesComponent do
+  let(:component) { Layout::CommonHTMLAttributesComponent.new }
+
+  it "includes the default language by default" do
+    render_inline component
+
+    expect(page.text).to eq 'lang="en"'
+  end
+
+  it "includes the current language" do
+    I18n.with_locale(:es) { render_inline component }
+
+    expect(page.text).to eq 'lang="es"'
+  end
+
+  context "RTL languages" do
+    let!(:default_enforce) { I18n.enforce_available_locales }
+
+    before do
+      I18n.enforce_available_locales = false
+      allow(I18n).to receive(:available_locales).and_return(%i[ar en es])
+    end
+
+    after { I18n.enforce_available_locales = default_enforce }
+
+    it "includes the dir attribute" do
+      I18n.with_locale(:ar) { render_inline component }
+
+      expect(page.text).to eq 'dir="rtl" lang="ar"'
+    end
+  end
+end