Re-arrange backup tasks to avoid holding db session open for too long.
Use delay/get to call tasks in backup tests.
Delete measures in batches of 10k rows in backup tasks.
Explicitly flush the session in between deletes.
Capture errors in sentry and retry all backup tasks.
Omit incomplete cell records from exports, closes #335.

This filters out all cell records which have no latitude set yet.
Explicitly allow anonymous data submissions, closes #41.

This just encodes the status quo more clearly. We allow access to the
submit API without or even with unknown API keys.
Remove the unused code for cell backfill, refs #17.

The current prototype for cell backfill of neighboring cell data was
never actually used in production. Let's remove it for now and revisit
this later with a fresh approach / code.
Updated to latest versions of dependencies.
Add a convenience `session.on_post_commit` helper method.

This makes it easier to schedule actions that should only occur once
a DB transaction has been successfully committed.