Code and partial tests for `LinksDb`

`LinksDb` represents the Links application database. It is initialized
with a backend that provide access to an actual database, and
communicates with that backend using the interface defined in
`test/interfaces/Backend.js`.

This commit includes tests of `LinksDb#ensureDb()`.
`RethinkDb`, which implements `Backend`, without tests
Notes on how to mock the RethinkDb driver
Removed errant, unused Buster.js dev dependency
Wrote tests for and fixed problems with `RethinkDb.js`