Export: Allow multiple simultaneous exports from a site

[rachelmcr]

We made a decision at one point to prevent multiple simultaneous exports, but that shouldn't be necessary. That said, if it's a real pain to allow simultaneous exports (from multiple sites as you switch around within the app, for example), we should think more about the best user experience.

If exports will rely on an async process and trigger a notification or other 'offline' alert, it seems fine to allow multiple exports. We don't have a great way to show progress, so the best we could do (if we prevent multiple simultaneous exports) is to disable the whole page and sort of just make people wait, which doesn't feel like a great experience.

We could trigger a global notice that looks like:

When finished it could fire off a success notice, and a notification that contains the link to download the export file, and an email saying the same.

If you trigger multiple exports you would have more than one 'exporting' notice stack up.

[stale]

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