DEV: Enable preserve_email_structure_when_styling by default

In 1bd8a075, a hidden site setting was added that causes Email::Styles
to treat its input as a complete document in all cases.

This commit enables that setting by default.

Some tests were removed that were broken by this change. They tested the
behaviour of applying email styles to empty strings. They weren't useful
because:

 * Sending empty email is not something we ever intend to do,
 * They were testing incidental behaviour - there are lots of
   valid ways to process the empty string,
 * Their intent wasn't clear from their descriptions,

spec/integration/email_style_spec.rb

@@ -8,8 +8,8 @@ describe EmailStyle do
     it "does not evaluate ERB outside of the email itself" do
       SiteSetting.email_custom_template = "<hello>%{email_content}</hello><%= (111 * 333) %>"
       html = Email::Renderer.new(UserNotifications.signup(Fabricate(:user))).html
-      expect(html).not_to match("36963")
-      expect(html.starts_with?('<hello>')).to eq(true)
+      expect(html).not_to include("36963")
+      expect(html).to include('<hello>')
     end
   end