Using tokens to hide parts of messages

We find that our users frequently leave the user-controlled part of the message blank, which ends up sending a message saying something like "Here is an important user from your friend:" with nothing after it.

This patch creates two special tokens, invite-if-message-start and invite-if-message-end, which will hide everything between them if there is no message.

To do this, it simply sets them to **HIDEME** and **ENDHIDE** respectively, then uses preg_replace to remove anything between those tags.

The patch also removes whitespace from the beginning and end of the message.