SessionManager: Add provision for encrypting session data

SessionManager: Add provision for encrypting session data

This follows the model Chris Steipp implemented for OATHAuth.

At the moment, this avoids the need to require a crypto PHP extension by
adding a configuration variable to enable plaintext storage. Someday
when there's time for the necessary code review, we should probably
import a pure-PHP implementation of AES to fall back to when the crypto
extensions are unavailable.

Change-Id: Ie9cae1526d3b8bf3f517f3226ddd888893f65656