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.html.erb

@@ -0,0 +1 @@
+<%= attributes -%>