Add pinned posts (#4217)

store/store.go

@@ -118,6 +118,7 @@ type ChannelStore interface {
 	InvalidateMemberCount(channelId string)
 	GetMemberCountFromCache(channelId string) int64
 	GetMemberCount(channelId string, allowFromCache bool) StoreChannel
+	GetPinnedPosts(channelId string) StoreChannel
 	RemoveMember(channelId string, userId string) StoreChannel
 	PermanentDeleteMembersByUser(userId string) StoreChannel
 	PermanentDeleteMembersByChannel(channelId string) StoreChannel