PERF: stop loading handlebars and ember compilers in prod

(this removes a nice 50K from our initial payload and saves memory)

Also fixes invalid HTML automatically if added to HEAD or /BODY

app/models/site_customization.rb

@@ -10,6 +10,10 @@ class SiteCustomization < ActiveRecord::Base
     %w(stylesheet mobile_stylesheet embedded_css)
   end
 
+  def self.html_fields
+    %w(body_tag head_tag)
+  end
+
   before_create do
     self.enabled ||= false
     self.key ||= SecureRandom.uuid
@@ -23,7 +27,32 @@ class SiteCustomization < ActiveRecord::Base
     raise e
   end
 
+  def process_html(html)
+    doc = Nokogiri::HTML.fragment(html)
+    doc.css('script[type="text/x-handlebars"]').each do |node|
+      name = node["name"] || node["data-template-name"] || "broken"
+      precompiled =
+        if name =~ /\.raw$/
+          "Discourse.EmberCompatHandlebars.template(#{Barber::EmberCompatPrecompiler.compile(node.inner_html)})"
+        else
+          "Ember.HTMLBars.template(#{Barber::Ember::Precompiler.compile(node.inner_html)})"
+        end
+      compiled = <<SCRIPT
+  Ember.TEMPLATES[#{name.inspect}] = #{precompiled};
+SCRIPT
+      node.replace("<script>#{compiled}</script>")
+    end
+
+    doc.to_s
+  end
+
   before_save do
+    SiteCustomization.html_fields.each do |html_attr|
+      if self.send("#{html_attr}_changed?")
+        self.send("#{html_attr}_baked=", process_html(self.send(html_attr)))
+      end
+    end
+
     SiteCustomization.css_fields.each do |stylesheet_attr|
       if self.send("#{stylesheet_attr}_changed?")
         begin
@@ -126,7 +155,12 @@ class SiteCustomization < ActiveRecord::Base
     val = if styles.present?
       styles.map do |style|
         lookup = target == :mobile ? "mobile_#{field}" : field
-        style.send(lookup)
+        if html_fields.include?(lookup.to_s)
+          style.ensure_baked!(lookup)
+          style.send("#{lookup}_baked")
+        else
+          style.send(lookup)
+        end
       end.compact.join("\n")
     end
 
@@ -142,6 +176,15 @@ class SiteCustomization < ActiveRecord::Base
     @cache.clear
   end
 
+  def ensure_baked!(field)
+    unless self.send("#{field}_baked")
+      if val = self.send(field)
+        val = process_html(val) rescue ""
+        self.update_columns("#{field}_baked" => val)
+      end
+    end
+  end
+
   def remove_from_cache!
     self.class.remove_from_cache!(self.class.enabled_key)
     self.class.remove_from_cache!(key)
@@ -190,6 +233,8 @@ end
 #  mobile_footer           :text
 #  head_tag                :text
 #  body_tag                :text
+#  head_tag_baked          :text
+#  body_tag_baked          :text
 #  top                     :text
 #  mobile_top              :text
 #  embedded_css            :text