Log actions taken

[adehnert]

Most actions generate emails, but without going and looking at those emails, it's hard to see what's been done. It'd be nice to also log events --- submitting vouchers, approving them, sending form emails, etc.. There should also be a UI for viewing the log --- including one for viewing all log entries, and one integrated with the request review page to show just the entries relevant to that request. (In particular, it'd be nice to be easily able to see when requests for documentation were sent.)