Allow square brackets in folder names

When an IMAP folder contains a ']' character (and the IMAP server does not escape the character), Geary would not be able to load the folder list, logging the warning "Received an unexpected closing brace" (see #1169 (closed))

In my case, Geary stumbled over a folder called "[Airmail]", which was automatically created by the Airmail app.

This patch tolerates a ']' in an atom if it would otherwise be an error.