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.

app/components/layout/common_html_attributes_component.rb

@@ -0,0 +1,17 @@
+class Layout::CommonHTMLAttributesComponent < ApplicationComponent
+  delegate :rtl?, to: :helpers
+
+  private
+
+    def attributes
+      sanitize([dir, lang].compact.join(" "))
+    end
+
+    def dir
+      'dir="rtl"' if rtl?
+    end
+
+    def lang
+      "lang=\"#{I18n.locale}\""
+    end
+end