FIX: Make Email::Styles operate on html documents instead of fragments

`Nokogiri::HTML.fragment` is a huge hack (a comment in the source code
admits this). The current behavior of `Email::Styles` is to try to
emulate `fragment` using nokogumbo, but it misses some edge cases. In
particular, meta tags in a email template don't make it through to the
final email.

Instead of treating the provided HTML as an indeterminate fragment, this
commit makes `Email::Styles` treat the HTML as a complete document. This
means that the generated HTML for an email will now always contain top
level structure (a doctype, html, head and body tags).

This new behavior is behind a hidden site setting for now and defaults
off.

config/site_settings.yml

@@ -1104,6 +1104,9 @@ email:
     client: true
     default: true
     hidden: true
+  preserve_email_structure_when_styling:
+    default: false
+    hidden: true
 
 files:
   max_image_size_kb:

lib/email/styles.rb

@@ -242,7 +242,12 @@ module Email
       strip_classes_and_ids
       replace_relative_urls
       replace_secure_media_urls
-      include_body? ? @fragment.at("body").to_html : @fragment.at("body").children.to_html
+
+      if SiteSetting.preserve_email_structure_when_styling
+        @fragment.to_html
+      else
+        include_body? ? @fragment.at("body").to_html : @fragment.at("body").children.to_html
+      end
     end
 
     def include_body?