Mail View: reuse the customized template, fix attachment icons

With the changes in `ftw.mail` (see b91f318) our customized mail-view template was not used any more.
Therefore it changed the view registration:
* Mail view: Use zcml instead of grok to register the og customized mail view. 

Additionally, i fixed the icons in the attachment listing:
 * Use sprite icons in the attachment listing.

@lukasgraf please take a look.