Add reporter's IP address

The report details in the MCP include:

1. report reason
2. reporter's username
3. report time
4. further information provided by the reporter
5. data about the reported post and its author.

While the IP address of the post author is displayed, the IP address of the reporter is absent (and it isn't apparently stored in the database).

That IP address can be useful, notably if the message is reported by a guest. That ability is disabled by default for anonymous, but it can be enabled and phpBB developers have expected a such case as a captcha appears when a guest wants to report a post (see template report_body.html).