[MM-36995] Start using db native JSON datatypes (#17930)

* Start using db native JSON datatypes

* Fix syntax to work on MySQL 5.7.12

store/sqlstore/thread_store.go

@@ -33,7 +33,7 @@ func newSqlThreadStore(sqlStore *SqlStore) store.ThreadStore {
 		tableThreads := db.AddTableWithName(model.Thread{}, "Threads").SetKeys(false, "PostId")
 		tableThreads.ColMap("PostId").SetMaxSize(26)
 		tableThreads.ColMap("ChannelId").SetMaxSize(26)
-		tableThreads.ColMap("Participants").SetMaxSize(0)
+		tableThreads.ColMap("Participants").SetDataType(sqlStore.jsonDataType())
 		tableThreadMemberships := db.AddTableWithName(model.ThreadMembership{}, "ThreadMemberships").SetKeys(false, "PostId", "UserId")
 		tableThreadMemberships.ColMap("PostId").SetMaxSize(26)
 		tableThreadMemberships.ColMap("UserId").SetMaxSize(26)