Update JQuery library and simplify OS detection

[chrisd8088]

The version of the JQuery library we use for the https://git-lfs.com home page was released in 2016 and is the subject of a few security reports, so although our use of JQuery is limited, we update it to the latest version. We also simplify our JavaScript code which performs a basic attempt at client OS detection.

First, we update our version of JQuery from is 1.12.4 to 3.7.1. The older version is the subject of a number of security issues, including:

• CVE-2019-11358
• CVE-2020-11022
• CVE-2020-23064

Next, we remove our use of the session.min.js JavaScript library, which appears to be from the codejoust/session.js project. Our version seems to match the code in the session.js file following PR codejoust/session.js#41, with minification applied manually. Regardless, as that project has not received updates in the last four years, and we only use a very small fraction of its functionality, we replace it with a few lines of CoffeeScript to perform a basic form of OS detection using the same technique as the codejoust/session.js library, namely a search for the specific strings Mac or Linux in the navigator.platform variable.

This variable's use is deprecated in current MDN JavaScript documentation; however, it remains the most straightforward
way of performing the basic and limited OS detection we require. In future we may choose to replace this with another approach, such as the User-Agent Client Hints API, but this is not yet supported by some major Web clients such as Firefox at the present time.

Both the legacy version of the codejoust/session.js library and the most recent update to that library (from 2019) pass the navigator object to the library's session_fetch() function, which calls the library'ssearch() function to set the os variable we check, passing an internal data.os array containing the possible OS types and their identifying strings. The search() function then tests for a match with one of these array elements, and in the case of the two checks we perform (for MacOS and Linux) this is just a search for a given substring (Mac or Linux) in the string returned by navigator.platform.

Thus we can just perform these simple checks ourselves, which will suffice for our purposes, and avoid the need to use a legacy JS library entirely. Our replacement CoffeeScript compiles to the following JavaScript:

$(function() {
    var platform;
    platform = window.navigator.platform;
    if ((platform.indexOf('Mac')) !== -1) {
        return $('.js-mac').removeClass('visually-hidden');
    } else if ((platform.indexOf('Linux')) !== -1) {
        return $('.js-linux').removeClass('visually-hidden');
    } else {
        return $('.js-windows').removeClass('visually-hidden');
    }
});

Finally, we can adjust the file permissions on one static file to remove its unnecessary executable file permissions.