The bottom scripts "site" and "user" currently use a very ugly synchronous script tag inside `document.write`.
Backstory: (Note that not all arguments are still relevant today, this is just for context.)
* They're not loaded with `mw.loader.load()` because these are legacy scripts that needs execution in global scope.
* We generally avoid ajax because of cross-domain in old browsers (though not a concern anymore for WMF since the demise of bits.wikimedai.org), because using XHR means we have to use eval, which means we lose useful stack traces.
* They're not a plain `<script>` tag because the request has to be conditional on `window.mw` - e.g. conditional on the browser being supported by MediaWiki and jQuery, and the startup request having succeeded without network or runtime problems.
* They're not a plain `<script>` and don't use `<script async>` because it needs a callback to update the mw.loader state machine and allow dependant modules to resolve.
* Unlike regular modules, we can't append a "state()" call to the server response because it's an unwrapped legacy script that may return early or throw exceptions, rendering the "state" call unreachable and causing the loader to hang indefinitely.