Include mail stylsheet tag in invoice pdf files and mailer layout [OFN-12214]

app/views/layouts/mailer.html.haml

@@ -5,7 +5,7 @@
     %meta{:content => "text/html; charset=UTF-8", "http-equiv" => "Content-Type"}/
     %title
       = Spree::Config[:site_name]
-    = stylesheet_pack_tag 'mail'
+    = stylesheet_link_tag 'mail'
   %body{:bgcolor => "#FFFFFF" }
     - unless @hide_ofn_navigation
       %table.head-wrap{:bgcolor => "#f2f2f2"}

app/views/spree/admin/orders/invoice.html.haml

@@ -1,3 +1,5 @@
+= wicked_pdf_stylesheet_link_tag "mail"
+
 %table{:width => "100%"}
   %tbody
     %tr{ valign: "top" }

app/views/spree/admin/orders/invoice2.html.haml

@@ -1,3 +1,5 @@
+= wicked_pdf_stylesheet_link_tag "mail"
+
 %table{:width => "100%"}
   %tbody
     %tr{ valign: "top" }

app/views/spree/admin/orders/invoice4.html.haml

@@ -1,3 +1,5 @@
+= wicked_pdf_stylesheet_link_tag "mail"
+
 %table{:width => "100%"}
   %tbody
     %tr{ valign: "top" }