TPAC: Subject headings should be schema:about, not schema:keywords

[Dan Scott]

* Evergreen master

Emerging practice has shown that http://schema.org/about is much better suited to descriptive entries such as subject headings instead of the more generic http://schema.org/keywords.

This is a minor adjustment to the TPAC templates.

[Dan Scott (denials)]

One-liner commit can be found at http://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/dbs/lp1419977_subject_headings_schema_about

[Dan Scott (denials)]

Make that a three-liner commit with much more granularity (one schema:about entry per subject heading, instead of the previous concatenated set of all subject headings under a single schema:about entry).

[Ben Shum (bshum)]

Pushed to master for 2.8 beta. Thanks Dan!