Add response header `X-Uncompressed-Content-Length` for JSON API

Because the response from the API is PHP output, the usual `Content-Length` header is absent.

This [custom header technique](https://stackoverflow.com/questions/15097712/how-can-i-use-deflated-gzipped-content-with-an-xhr-onprogress-function/32799706#32799706) allows the client to know the total length of the data being received, in order to display a progress indicator.

Here's a code example with `XMLHttpRequest`:


```
xhr.addEventListener("progress", (e) => {
	if (e.lengthComputable) {
		onDownloadProgress({
			loaded: e.loaded,
			total: e.total,
		});
	} else {
		const uncompressedContentLength = xhr.getResponseHeader(
			"X-Uncompressed-Content-Length",
		);

		if (uncompressedContentLength) {
			onDownloadProgress({
				loaded: e.loaded,
				total: Number(uncompressedContentLength),
			});
		}
	}
});
```

Notes:
- `Fetch` can be used as well (only reason I use `XMLHttpRequest` is because `fetch` doesn't allow to track the progress of uploaded data (when creating a paste); whereas `XMLHttpRequest` does).
- `e.loaded` can be different between browsers; Firefox reports the length of the compressed data, Chrome reports the length of uncompressed data (see https://github.com/whatwg/xhr/issues/388). A workaround for this is to manually set our progress indicator to 100% when the request finishes.

lib/Controller.php

@@ -151,6 +151,8 @@ class Controller
             header('Access-Control-Allow-Origin: *');
             header('Access-Control-Allow-Methods: GET, POST, PUT, DELETE');
             header('Access-Control-Allow-Headers: X-Requested-With, Content-Type');
+            header('X-Uncompressed-Content-Length: ' . strlen($this->_json));
+            header('Access-Control-Expose-Headers: X-Uncompressed-Content-Length');
             echo $this->_json;
         } else {
             $this->_view();