Process request only with OAI API key. refs #7469

Added a setting to disable OAI API key authentication

Added logic to require OAI requests provide key if an OAI
key exists and OAI key authentication is enabled. Sign in
user if correct OAI API key supplied so ACL checks in
template work.
Include about tag in OAI list output. refs #7468

Created a separate template for the about tag and made it so
the about tag is included in OAI-PHP output for the listRecords
verb (as well as getRecord).
Replaced primary key with slug. refs #7468

Replaced the use of a digital object's primary key with its
slug.