When forwarding an email, pre-load its attachments.

When forwarding an email, pre-load its attachments.

This completes the effort to server-side-preload the attachments from
the original message.  They appear as attachments in the forwarded copy
of the message in the composer, using the same API.  A lot of effort
went into making the 'attach' and 'forward attachments' functions DRY
with respect to each other.