0020_nextcloud_mozilla_pre: auto-provision Thunderbird IMAP account at logon

Fetches user_full_name (givenname + sn) and user_email from FreeIPA via
ipalib and writes them into the Thunderbird IMAP account prefs. Adds
ipalib availability check to logon_script.sh. Drops TB_MAIL_FULLNAME
config variable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/setup_system.conf.dist

@@ -80,7 +80,6 @@ if [ "$EUID" -ne 0 ]; then
 
   # Mail account auto-provisioning for DAVTOKEN_USER@TLDOMAIN in Thunderbird
   export SERVERFQDN_IMAP="imap.${TLDOMAIN}"  # IMAP server hostname (e.g. imap.strato.de)
-  export TB_MAIL_FULLNAME="${USER}"           # Display name written to the Thunderbird identity
 fi
 
 #Basic commons not needing change