api.email.getThread fetches only the newest 50 messages of a thread
(Thread/get, then Email/get for those ids) instead of all of them with
their bodies. api.email.getThreadCount answers the number of messages
from Thread/get, with its own rate limit.
plugin-host-thread.test.ts now expects getThreadEmails to be called with
{ newest: 50 }, the new option that limits what is fetched.
doEmailGet asked the signed-in account for every id, so a message in a
folder shared from another account came back as null even though it was
in the list. A message in the loaded list is now read from the account
that owns it, through resolveEmailBlobContext like attachment downloads.
getAttachment and getRaw keep reading from the signed-in account only.
Fixes#1273
Linus Rath
Ali Yousuf