Change ViewAttachmentServlet to make use of AttachmentManager

All attachment handling inside JIRA goes over the AttachmentManager (create,delete,persistOnFileSystem and so on) but the "View Attachment" operation didn't make use of the AttachmentManager, it implements his own logic to deliver the attachment to the viewer inside ViewAttachmentServlet. This is not clear, if implementing an own AttachmentManager extenting the AttachmentManager through a Plugin is not enough, it is also necessary to override/change the ViewAttachmentServlet, but extending JIRA doesn't have to require code changes to JIRA CodeBase.