LP#1842297: Implements patron sign-on to the OpenAthens service.

LP#1842297: Implements patron sign-on to the OpenAthens service.

Allows global or local administrators to configure a connection to the
OpenAthens cloud-based single sign-on service. Allows patrons to connect
seamlessly to third party resources that use OpenAthens authentication.
For more detailed feature description visit
https://wiki.evergreen-ils.org/doku.php?id=dev%3Aproposal%3Aopenathens_integration

Signed-off-by: Julian Clementson <julian.clementson@openathens.net>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>