Emails: add support for attachments

[muratcakir]

Also add options to attach order specific PDF files automatically to an order notification email (leveraging the new HTML to PDF converter)

[muratcakir]

Also required for new import/export infrastructure

[muratcakir]

• Add Attachments to Message Templates (referencing regular Downloads/Files)
• New entity: EmailAttachment (1:n: QueuedEmail > Attachments)
• EmailAttachment either holds reference to Download or the actual file binary
• To add dynamic attachments (like OrderConfirmation), a plugin should react to applicable events, create the file (wkhtmltopdf) and add it to the attachment list.