Session management

[lullis]

Fixes #1130

Description of the Change

Implementation of OIDC Session Management. This PR:

• Adds some settings variables to enable OIDC Session Management
• Adds checks to validate OIDC Session Management settings - if it is enabled, then Django will require a OIDC_SESSION_MANAGEMENT_DEFAULT_KEY to be present.
• Adds the session_state parameter to the parameters given to the end user client upon succcessful authorization. The session state is calculate by a salted hash combination of client id, origin and "OP user agent state", which for the intents of session management is itself just a hashed value of the session key for authenticated users, and OIDC_SESSION_MANAGEMENT_DEFAULT_KEY if the user is not authenticated. This is enough for the OP to indicate whether the end user session has changed (logged in, logged out)
• Add a OIDCSessionManagementMiddleware that sets a cookie with the value of "OP user agent state"
• Adds a SessionIFrameView endpoint for the "OP IFrame"

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS