Handle with care mailbox names ending with separator

Some servers apparently find funny to sometimes send mailbox names
suffixed with the separator (e.g. INBOX/) which mapped for us to a
mailbox with no name under INBOX. Needless to say it was veeery wrong.

So now special case those, and consider that the server instead meant
the mailbox name without this separator suffix (e.g. INBOX/ -> INBOX).

BUG: 258342