Linux client - merge branch 'develop'

the-linux-schools-project/karoshi-client@5ddb400c26666d5c6eda17835bd4164434f9c720

Features:
 * Thunderbird will not autostart if a mail server is not configured
 * Support for user web folders, with generation of icon on desktop
 * MIME type associations set correctly for various files, notably XML
 * SSH Kerberos delegation support is integrated

Notes:
 * SSH will not be installed by default until web management support is there
 * Autogeneration of firewall rules has been preserved to avoid breaking
   existing systems