Transmission: Enable single sign on

Created by: prachisr

Complexity: 2 points Assumption: Framework for creating LDAP user groups is implemented in #928 (closed). Assumption: The user group 'bittorrent' is always shared Deluge. See #948 (closed).

• Remove authentication on transmission web interface.
• Remove configuration fields for username and password from the Transmission application page in Plinth.
• Enable single sign on by reusing configuration already available as web server configuration include file. See other applications that use mod_authpubtkt for authentication for examples.
• Create a user group in LDAP called 'bittorrent'.
• Restrict access to user group 'bittorrent'.
• Update the manual page about the authentication mechanism.