Make Export of statistics/annotations more responsive

[StephenChan]

Currently on a large source, you'd click one of the buttons on the export page, and your browser would start spinning as it waited for the request to finish. For a decent amount of data, it takes long enough that you could grab a cup of coffee and a snack (as the page used to say). I imagine there are also sources where you'd wait until you woke up the next morning. All the while, the browser tab is still spinning and there is no indication of when it'll finish.

• [ ] The export request should be an Ajax request, so that the browser window doesn't say "waiting for response..." the entire time.
• [ ] There should be a progress indicator on the page, perhaps based on number of images processed. A text percentage at minimum; preferably a progress bar too.
• [ ] See if we can speed up the export functions. I'd start by checking the number of database queries involved.

[StephenChan]

Export has now been moved to Browse, but it's still non-Ajax and has not undergone performance analysis.

[beijbom]

Seems with the move to the image browse page we no longer warn users the export might take a long time. I think we need to add that back in.

[StephenChan]

Well, the warning's hidden away in the '?' button. But yeah, it should be more prominent again.

(Edit - Added back in: 321b8170ccb3d3c141c5a3ff1c0d5263fac1b2ec)