Let users inject js/html on to new tab page

[bovine3dom]

As discussed with gamingjones223 on matrix

Users could display weather, bookmarks, fortune | cowsay whatever

Part of me thinks I could make this a "premium" feature for donors with some barely-secret password to unlock it since I don't really want to implement it and it feels out of scope for Tridactyl, and also since, depending how we implement it, it could hide the donation reminder which is literally how I afford to eat. And I enjoy being able to afford to eat maybe even more than other people enjoy customisable new tab pages