Throttle Requests from TOR

[v]

Pretty version of this exists at https://trac.rutgers.edu/trac/roundcube/ticket/26

A plugin should be written that interfaces with the TOR DNSEL. See

https://www.torproject.org/projects/tordnsel.html.en

for details. The configuration options should be:

DNS root to query (i.e. "ip-port.exitlist.torproject.org")
OPTIONAL HTML text to display if a TOR match is detected. This should come in as a modal upon login; see for example the convert_userprefs plugin. Default:

"You logged in using an anonymous proxy. But you just gave us your identity via your username and password. Not very anonymous, huh? So we're only going to let you send one message per hour. Drop the proxy...it's not helping you anyway."

If this option is not set, no modal dialog, just log right in.
Override roundcube config values if a TOR host is utilized. In theory this should let you override anything. But if there's some need to be explicit, we want to override:

// How many seconds must pass between emails sent by a user
$TOR_rcmail_config['sendmail_delay'] = 3600;
// Maximum number of recipients per message. Default: 0 (no limit)
$TOR_rcmail_config['max_recipients'] = 5;
for the TOR users.

Whether or not to send alert emails, with specifications and configuration directives identical to the Rutgers geoip plugin.