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

Related issues

• Related to #16087 (closed)
• Has duplicate #16087 (closed)

Original created by @u on 16098 (Redmine)