Properly decode Unicode folder names; fix #5217
Previously, we were taking folder names as they came off the wire. Turns out IMAP specifies that folder names with 8 bit code points are encoded in a crazy scheme unique to IMAP. Now, we properly decode that scheme to the correct UTF-8 folder names to be displayed to the user. There's also now a database upgrade path that converts all existing mailboxes to the decoded version, so your existing database should just keep working.
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