In Matt 21, he was talking to those disciples that were with him. He was giving them a lesson about faith and doubt and that they can see those same results.
In Luke 17 it would appear he was also talking directly to his disciples but there were a lot of people about the place.
Is that the end of the story? no.
Does Jesus suggest that we can do those miracles today? I believe he does
Firstly, the 12 original Apostles were not the only ones doing miracles. There were others who were sent out to do so. See Luke 70. Jesus commisions 70 to go out and... "And heal the sick that are therein, and say unto them, The kingdom of God is come nigh unto you." - Luke 10:9. And they did because they came back with a good with Joy saying even the devils are subject to them.
However, John 14 makes it pretty clear. The only qualifier is believe on Jesus. What were the works he was doing? healing the sick, casting out devils, raising the dead.
12 Verily, verily, I say unto you, He that believes on me, the works that I do shall he do also; and greater works than these shall he do; because I go unto my Father.
13 And whatsoever you shall ask in my name, that will I do, that the Father may be glorified in the Son.
14 If you shall ask anything in my name, I will do it.
Also the great commission in Mark 16 "go into all the world"... The only qualification is that signs will follow those who believe. Some would argue that he's saying "those disciples" that were with him. In that case, why preach the gospel since by that logic Jesus only told those disciples to do so. No this is for those that believe.
17 And these signs will follow those who believe: In My name they will cast out demons; they will speak with new tongues; 18 they[a] will take up serpents; and if they drink anything deadly, it will by no means hurt them; they will lay hands on the sick, and they will recover.”
There is also an argument going around that says we have the whole bible so therefore we don't need miracles today. And the bible verse that is used to back that up is:
1 Corinthians 13:8 - "Charity never faileth: but whether there be prophecies, they shall fail; whether there be tongues, they shall cease; whether there be knowledge, it shall vanish away. 9For we know in part, and we prophesy in part. 10But when that which is perfect is come, then that which is in part shall be done away."
The claim is that that which is perfect is come and his name is Jesus and he's said on the cross - it is finished, therefore miracles are no longer needed. However, this doesn't make sense. Tongues have not ceased, prophesies are not failing and I'm sure everyone would agree knowledge is actually on the increase.
Nope, miracles are definitely for today and I've witnessed some big ones.
I should clarify. Without God we're nothing. We can't do anything. It's God doing the miracle, but he uses people. He works through people.