Mail view: Use zcml instead of grok to register the og customized mail view.

With the changes in `ftw.mail` (see b91f31810bd19f3bd994b960490515d03b162aca)
our customized mail-view template was not used any more.
Mail view: use sprite icons in the atachments list.
Merge pull request #244 from 4teamwork/pg_mail_template

Mail View: reuse the customized template, fix attachment icons