Schedule report tasks and add report preview view

[GregKaleka]

• Significantly improved the speed of the "Do Everything" (DE) button by restricting the data imports to the most recent two versions (for diffing), when before they were importing everything, taking hours. Should now take less than an hour to complete.
• Added actual report generation and caching to the DE button (this takes only a few extra seconds)
• Added a weekly scheduled task that runs the DE process
• Added a url and view for staff only that shows the cached report for a given version if it exists, available at /releases/<version_slug>/report

Note: I have not implemented PDF rendering. I can do so, but wanted to have some discussion about what library to use. I've used WeasyPrint in the past, and the results have been good, though I recall the dev experience being annoying (someone on the team was always having to install some supporting thing on their machine).

Fixes #1603