Bug 53105: Make the extension function independently from the schema.

This patch makes the extension less reliant on the schema looking
exactly like the one at
https://meta.wikimedia.org/wiki/Schema:BookManagerv2. It also
formats the dates more nicely, with a couple of requirements for
the schema, which I will document shortly.

Change-Id: I4d9faa38b2d74daf7c787b6ca163bd4d862d7e30
Merge "Bug 53105: Make the extension function independently from the schema."