roundcube: breaking configuration changes in 1.6.0
https://github.com/roundcube/roundcubemail/releases/tag/1.6.0
FreedomBox is affected by the following changes in Roundcube:
- renamed default_host to imap_host
- set "localhost:143" as a default for imap_host
- renamed smtp_server to smtp_host
- removed smtp_port option (non-standard port can be set via smtp_host)
- set "localhost:587" as a default for smtp_host