Include a full-page screenshot (instead of visible part of the screen)

[basilesimon]

We're including a base64-encoded screenshot PNG which, disappointingly, is only the visible part of the screen (see [browser.tabs.captureVisibleTab](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/Tabs/captureVisibleTab)).

Moving to a full-page screenshot will involve some fiddling with simulating a scroll while capturing with Screen Capture API, I'm told.

[nikonikoniko]

firefox itself seems very capable of full page screenshots: https://screenshots.firefox.com/

[padenot]

This is implemented around here. There is a bit of documentation, read the disclaimers: https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/drawWindow.