Add a downloads page, listing all the files in the export/ bucket.
Add a mocked functional test for listing downloads.
Cache the S3 download list lookup in Redis for 10 minutes.
Add changelog and enable cell export tasks, closes #295.
Add explicit CC-0 license note to downloads page.
Merge pull request #299 from hannosch/295-add-downloads-section

Add downloads section to website and enable export tasks