 Improve default layout for PDF exports and make it customizable

PDF export currently uses a dummy layout, we should:

- provide a "good-looking" default
- failsafe internationalized page numbering and localized date (Created: ...) support
- optionally provide a "even-better-looking" default with a nice front page
- define a supported default way to create "custom logos" (keeping "powered by Icinga") and "custom layouts"