add grabber for web (twitter)

[ethernian]

create a Grabber for Twitter implemented as a new kind of Grabber: the Web Grabber (with the Twitter configuration).
The web grabber should fit into the existing Core. Every necessary extension of the core infrastructure should be made explicit.

Requirements and Assumptions:

1. The Web Grabber is a generic grabber to grab any website. Any specific website grabbing details is to be stored in the grabber's configuration.
2. Different website grabbers (like Twitter or TelegramWeb) are different instances of the same Web Grabber, but launched with website-specific configurations.
3. The Web Grabber should fetch specific website configurations automatically.
4. The Web Grabber is assumed to reuse the Mutable Web architecture.
5. The Web Grabber is supposed to run inside an automated headless browser.

ToDo:
Breakdown into meaningful steps.