Implement some channel endpoints for APIv4 (#5767)

store/store.go

@@ -134,6 +134,7 @@ type ChannelStore interface {
 	SearchMore(userId string, teamId string, term string) StoreChannel
 	GetMembersByIds(channelId string, userIds []string) StoreChannel
 	AnalyticsDeletedTypeCount(teamId string, channelType string) StoreChannel
+	GetChannelUnread(channelId, userId string) StoreChannel
 }
 
 type PostStore interface {