Homestyx hydra

R1:0a335f91cdfb

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.
Repository: R1 hydra
Commit Date: Apr 19 2017