Combine jQuery and jQuery migrate to reduce HTTP requests

[peterwilsoncc]

Combining the jQuery and jQuery migrate scripts will improve performance by reducing HTTP requests. As jQuery scripts are loaded in the header, this will reduce the period of render blocking.

In the attached patch

• the files load separately when SCRIPT_DEBUG is enabled
• jquery-core and jquery-migrate effectively become aliases of jquery
• aliases are queued to footer to allow theme/plugin devs to move jQuery to footer if they wish

Cons/risks

• plugins/themes replacing jquery with a CDN version could remove migrate
• drama and pushback from devs who keep their code free of deprecated jQuery functions

It has been pointed out that jQuery-migrate can introduce vulnerabilities fixed in past versions of jQuery.

​http://wphighed.org/2014/10/potential-cross-site-scripting-vulnerability-in-jquery-scripts/

Should we instead look at a way to reduce the need for jQuery-migrate and get everybody to use the most recent versions, without old dependencies?

Replying to Otto42:

Should we instead look at a way to reduce the need for jQuery-migrate and get everybody to use the most recent versions, without old dependencies?

#22975 and #25371 (and I'm sure others) dealt with removing deprecated functions from core. Including a test for deprecated functions in grunt would be a useful addition to the core build process.

It would be nice to encourage plugin and theme developers to use jquery-core but it does run the risk of abandoned plugins blowing up sites' JavaScript. But yes, it would be useful.

Related: #32358

Refresh reqd following #32794 [32978]

32793.2.diff contains the refresh for jQuery 1.11.3