Feature: Allow external PDF generation instead

[jbostoen]

Typical webhosts (PHP + database setup) don't allow customers to set up the required components (e.g. install a headless Chrome, NPM, Puppeteer, ...).

A default ReportProcessor option should be to send the HTML data to an external service, which renders the PDF instead.

• Allow user to choose between (internal) BrowserShot PDF rendering or using an external service
• Allow some options, e.g. skip certificate check

Implemented (experimental) with 526bc48