roundcube: 1.3.7 -> 1.3.8

This is a service release to update the stable version 1.3 of Roundcube
Webmail.  It contains fixes to several bugs backported from the master
branch including a security fix for a reported XSS vulnerability plus
updates to ensure compatibility with PHP 7.3 and recent versions of
Courier-IMAP, Dovecot and MySQL 8. See the complete changelog at
https://github.com/roundcube/roundcubemail/releases/tag/1.3.8