Remove "participationStatus" from ConpherenceParticipant

Summary:
Pathway to D17685. This column is a very complicated cache of: is participant.messageCount equal to thread.messageCount?

We can just ask this question with a JOIN instead and simplify things dramatically.

Test Plan:

• Ran migration.
• Browsed around.
• Sent a message, saw unread count go up.
• Read the message, saw unread count go down.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17730