T590 availability 

Lots of changes here. I've added an API call /availability?bibid=12345 which returns the availability for a resource as JSON using [schema.org's holdings support]. The search results page uses this API to look up the availability for results via AJAX. You can pass in the fake bibids (GM and GT) that are in Summon.

Two important points:

* only availability for the given bibid is returned. so it doesn't currently fan out to similar bibids. if the similar bibids are known they can be looked up individually.
* status from Georgetown and GeorgeMason isn't available yet, since z39.50 hasn't been rolled in yet.

If you take a look at the implementation you'll see the api call just calls `db.get_availability`.  I've continued to pull bits of `ui.voyager` over to the new `ui.db` module and to test them in `ui.dbtest`. Eventually the older code can be removed once everything has switched over.

This work isn't finished but I'd like to open up separate tickets for the work that remains, and add them to the next iteration.

fixes #590