Update facility data OAI-PMH metadata to record information as items

Motivation:

The current OAI-PMH information is recorded as `datasets`.  However,
this assumes that the items underlying the harvested OAI-PMH records
correspond to datasets.  This is not guaranteed, and there are
counter-examples.

OAI-PMH describes three concepts: resource, item and record.  The
OAI-PMH responses provide records (descriptive metadata of some item) or
identifiers thereof.  However, since OAI-PMH requires repositories to
support Dublin Core records, it seems a reasonable assumption for there
to be a 1:1 relationship between each Dublin Core record and some
corresponding item.

Therefore, we can use the metadata-agnostic `item` concept when
describing the information about the endpoint.

Modification:

Update script to record information under `items` node in the facilities
YAML file.

Update the Jekyll to consume this information when rendering the
corresponding HTML.

Result:

No observable change, but the facilities YAML file now uses the more
neutral 'items' instead of 'datasets'.