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
Add Redis based caching for SQL queries used in the website.
Initial sketch of OpenCellID import code, cc #283
Support and test OCID cells in ichnaea.service.locate code path, cc #283.
Address review comments, cc #283.
Add support for importing already-existing rows as updates, close #283.
Address review comments, cc #283.
Remove AUTO_INCREMENT=34 from alembic sql.
Accept float ranges in import, convert to int.
Skip header row on import.
Commit each batch of imported values as they are imported.