Add "IMAP folder" field to advanced relay settings

IMAP login params have a "folder" setting since 2.50, right after the "port" setting. It is a replacement for old `only_fetch_mvbox` setting, for migrated users it may be set to `DeltaChat`. It should be displayed, possibly as a disabled field, and resubmitted when user reconfigures the relay so the relay does not get reconfigured to use INBOX folder accidentally just because user wants to change the password.