Add UniqueConstraint to custom through tables

[amickan]

As per the Django documentation, we should add a UniqueConstraint including the from and to fields to all custom through tables in the codebase.

List of through tables that need updating:

• OptionalHangingProtocolAlgorithm
• OptionalHangingProtocolArchive
• DirectMessageUnreadBy
• ConversationParticipant
• OptionalHangingProtocolPhase
• CombinedLeaderboardPhase
• WorkstationSessionReaderStudy
• OptionalHangingProtocolReaderStudy
• VerificationUserSetUser

[chrisvanrun]

Not all of them, just the ones that weren't automatically created by Django for some relations.

[amickan]

Those are all the custom through tables.