* Handle messages >25k
* When structure is available (e.g. IMAP) pull in the entire body
and the list of attachments
* When structure is not available (e.g. POP) pull in a large chunk of
the body to try and capture the message body at least.
* Implement loadAttachment for IMAP/POP to demand download large items
* Tested with IMAP & POP messages
INCOMPLETE (file bugs):
* implement logic for the old loadMessageForView calls that comes from
MessageView (when you open a message that's partially-loaded)
* Resolve handling of mimetype when attachment info is read (currently
we're assuming base64 in a couple of places)
* delete account => delete attachments
* delete attachment => delete file
* create account => clear existing attachments for acct id
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