403 error when refocusing tab with private document

[allanlasser]

I had an open browser tab for a private document, which I left open but inactive during a meeting. When I returned to the tab, the viewer showed a 403 error. The key had expired while the tab was inactive, but refocusing the tab didn't rerender the page, so the token was stale and did not refetch.

[eyeseast]

@mitchelljkotler ran into this when viewing a private document. He was logged out and got a 404, then logged in and got a 403.

[eyeseast]

Cloudflare is caching the JSON response when we fetch. Looks like we need to add some no-cache headers on the JSON response here: https://github.com/MuckRock/documentcloud/blob/master/documentcloud/core/views.py#L38.