Make speaker permission to create sessions an access policy

Problem/Motivation

Currently this project broadly gives speakers the ability to create sessions. In practice, however, speakers should only be able to create sessions after the "call for session proposals" has started, and before it closes. Since the implementation of an event planning workflow we can map this to the state of the current event, and only allow speakers to submit sessions while the current event is in the Sessions Open state.

Proposed resolution

Remove the broad permission for speakers to submit sessions and create an access policy to allow speakers to submit session while the current event is in the Sessions Open event planning state.