Notify users that have payment overdue

Notify only once.
housekeeping
PaymentOverdueNotifier: account for dev mode
housekeeping
Store search history in Firebase too
Another partial rewrite

This was triggered by the need to send payment overdue notifications,
and ended up as a larger rewrite, one which I am happy with.

Changed modules from nouns to actions. This makes the naming much more
natural and I don’t have to invent nouns like “inquirer”,
“email-sender”, and “payment-overdue-notifier”.

Elimintated maxcomplexity thing from JSHint because it made noise in a
couple of places where it was not appropriate.

Wrote a few tests and then deleted them all because the logic kept
changing three times during this rewrite and I was becoming inconvenient
to update tests.

Moved search history storage to Firebase.

Rethought custom field computation.
Data.getLastChildOf: return {k:K,v:V} instead of V
SearchHistory.getPreviousResults: timestamp
Record search timestamp
.gitignore: quick experiments
Skip users that were already served today
mv app/data.js app/util/data.js
mv app/prepare-email-bodies.js app/prepare-news-email-bodies.js
mv app/send-email.js app/util/send-email.js
housekeeping
housekeeping
a little optimisation
mv app/util/get-section-columns.js app/util/get-useful-section-columns.js
mv secrets.json app/secrets.json
v 2.0.0