Inline installation deprecated in Chrome 71
Our download.js in https://tails.boum.org/install/debian/usb-download/index.en.html uses `chrome.webstore.install();`
It will be impossible to install the extension like that starting in December 2018: https://blog.chromium.org/2018/06/improving-extension-transparency-for.html (with the release of Chrome 71).
As suggested in the blog post, we “simply” need to replace this piece of code with a link to https://chrome.google.com/webstore/detail/tails-verification/gaghffbplpialpoeclgjkkbknblfajdl.
Feature Branch: tails.git:bug/16098+Chromeinstall