Sends information about a topic to a friend.

• Topic is set via $topic global variable.
• Uses the SendTopic template, with the main sub template.
• Requires the send_topic permission.
• Validates the form data if $_POST['send'] is set and throws an error if something isn't valid
• Sends the email if the form data is valid
• Redirects back to the first page of the topic when done.
• Is accessed via ?action=sendtopic.