Added features to OAI-PMH API. refs #7469

Added embedding of digital objects in records returned
by OAI-PMH queries. Added API key authentication.

Digital objects are embedded in the <about> tag at an
Atom XML feed.

API keys can be generated for individual users, via the
user admin interface, and optionally required for authentication.

Smaller changes:
* Removed OAI disabled page.
* Got rid of redundant OAI-related setting.