Fix tests to use zero padding on returned years
Accept date label "present"
Parse BP dates with different bases than 1950
Change logic in date formatting to account for year 0

since 0 is falsy
Ordinal number words should be case-insensitive
Start adapting period form to handle ranges & expanded parser

Especially, don't worry so much about the date types parsed
Update parser tests

Most importantly, BP base should be 1950 by default
Use WorldCat JSON-LD resources instead of Turtle

The latter didn't have CORS enabled so we were going through a proxy on
my own domain. JSON-LD resources do right now, so we'll use them. It's a
pain to parse two different types of linked data, but actually not as
much of a pain as I'd thought!
Change "periodizations" to "period collections"
Handle sources that only have titles as part of another work
Improve handling earliest/latest endpoints; use Tablesort
Fix field binding for new periodizations in edit form