Don't allow mailboxes named "." or ".." and normalize names during imports too

It only serves to confuse. When exporting such mailboxes in zip files or tar
files, extracting will cause trouble.

webaccount/import.go

@@ -433,10 +433,10 @@ func importMessages(ctx context.Context, log mlog.Log, token string, acc *store.
 	}
 
 	xensureMailbox := func(name string) *store.Mailbox {
+		// Ensure name is normalized.
 		name = norm.NFC.String(name)
-		if strings.ToLower(name) == "inbox" {
-			name = "Inbox"
-		}
+		name, _, err := store.CheckMailboxName(name, true)
+		ximportcheckf(err, "checking mailbox name")
 
 		if mb, ok := mailboxNames[name]; ok {
 			return mb