Change "Published" state to "Confirmed" in the Session Acceptance workflow

Problem/Motivation

The current "Published" workflow state in the Session Acceptance workflow won't necessarily be used. It could be more useful if we changed the label to Confirmed, and added an ability for speakers to make a transition to this state by confirming they will present their session.

Proposed resolution

• Change the label for the state to Confirmed
• Consider adding a "declined" stage for sessions that a speaker is not able to deliver
• Include new transitions, and add permissions for speakers and session moderators to perform them