so users can easily take their email out of somewhere else, and import it into mox.
this goes a little way to give feedback as the import progresses: upload
progress is shown (surprisingly, browsers aren't doing this...), imported
mailboxes/messages are counted (batched) and import issues/warnings are
displayed, all sent over an SSE connection. an import token is stored in
sessionstorage. if you reload the page (e.g. after a connection error), the
browser will reconnect to the running import and show its progress again. and
you can just abort the import before it is finished and committed, and nothing
will have changed.
this also imports flags/keywords from mbox files.
named "traceauth" and "tracedata".
with this, you can (almost) enable trace logging without fear of logging
sensitive data or ddos'ing your log server.
the caveat is that the imap login command has already printed the line as
regular trace before we can decide it should not be. can be fixed soon.
