refreshment/last_update dates not handled correctly

This issue collects a couple of related things about the handling of last refreshed/last updated dates in scoreboard and osmesa:

1. In [api/src/services/osmesa.js](https://github.com/developmentseed/scoreboard/blob/develop/api/src/services/osmesa.js#L383-L386) `getUpdates()` `Date.now()` is returned if the osmesa `refreshments` table is empty. This could (possibly) lead to a situation where scoreboard ui is displaying the current date is shown but really it is unknown if/when the database last updated. It would be better to throw an exception and let the UI display an empty value or error message.

2. The the dev environment, the osmesa `refreshments` table is empty, as is the scoreboard `user_update` table. In #530 I listed some sql queries to populate these tables for testing in development. These tables should be added to the seed data in scoreboard and in https://github.com/developmentseed/osm-fabricator so it just works in the dev environment.

Start with getUpdates() in api/src/services/osmesa.js and review the SQL queries from issue #530. Then locate the seed data in scoreboard and osm-fabricator. Done means unknown refresh dates are shown as empty or an error rather than the current date, and both development tables contain seed data.

Written by the indexing model from the issue text.