Add SpecialPage for Electron PDF rendering

This adds functionality to render an article with Electron service.
If Collection extension is installed it replaces the "Download as PDF"
link in the sidebar otherwise adds a new link to the sidebar.

Still to do (in follow-ups):

• Replace the "Printable version" link in the sidebar
• Add UI to let the user choose between OCR and Electron rendering
• Make sure the link is only active for articles
• Make additional checks to only render valid article links
• etc..

Bug: T142201
Change-Id: Ie0d4794f9b6ae1f441a1e527fec872d9e228ad10