Uploader: return an ID not HTML from async-upload

[tellyworth]

Currently, async-upload.php returns a big chunk of HTML on a successful upload, containing the form for filling in the title, caption etc. It runs to around 4800 bytes.

I've run into a situation where (for reasons unclear) ajax responses are truncated to about 2.5kb. The enclosed patch changes async-upload.php to fix this: instead of HTML it returns the attachment ID only on a successful upload. The HTML form is fetched in a separate ajax request. The result is the same, and -- truncation aside -- I think this is a slightly more elegant and maintainable approach.

[tellyworth]

This has a bug. Working on it.

[tellyworth]

I've created a new ticket with a better explanation of the problem. It's a serious bug that needs fixing in 2.5.1.

See #6713.