Provide a web UI for export queue

[Dafydd Harries]

In bug #2038, Zygmunt Krynicki suggested that we have a page where you can see the export requests you made. I really like this idea. For each export, we should show:

 - what state it's in
 - if it succeeded, the Librarian URL for the completed export
 - if it failed, the error message for the translation

Since the error messages can be quite long, perhaps those should be displayed on a separate page.

This would also make automated exports easier, even if it requires screen-scraping.

[Carlos Perelló Marín (carlos)]

I agree with you, after working on improve the translation import queue, I thought we could create the same kind of page for the exports so I suppose it's a matter of reuse some code from there.

[Matthew Paul Thomas (mpt)]

As I wrote in bug 162771: "As shown in bug 160551, this causes people to request exports repeatedly because they don't know what happened to their previous request."

[Jeroen T. Vermeulen (jtv)]

An ability to cancel requests would also be most useful.