Improve support for multiple speakers

Problem/Motivation

Sessions currently support specifying multiple speakers, but if used this creates problems. For example, views can show duplicate rows where multiple speakers have been set.

Proposed resolution

Ensure that displays, including views, are optimized to properly handle sessions with multiple speakers. This may also be an argument in favour of replacing Workbench Email with ECA, since that switch would make it easier to send emails to all users indicated as speakers, and not just the node owner.