Convert feed format to Atom

Converted feed output to Atom, also added content type header for the json output.
This format works well with services like Slack.
@stipsan For full backwards compat, the block at line 259 can be removed, and this block should be changed to:

```php
if ( 'atom' === $format ) {
	require_once WP_STREAM_DIR . 'ui/feeds/atom.php';
} elseif ( 'json' === $format ) {
	require_once WP_STREAM_DIR . 'ui/feeds/json.php';
} else {
	require_once WP_STREAM_DIR . 'ui/feeds/rss-2.0.php';
}
```