Use the new NIP17 for encryption on DMs

Right now we use [NIP4](https://github.com/nostr-protocol/nips/blob/master/04.md) for sending encrypted signatures between founder and investor, that NIP is deprecated we must instead use [NIP17](https://github.com/nostr-protocol/nips/blob/master/17.md) and [NIP44](https://github.com/nostr-protocol/nips/blob/master/44.md) / [NIP59](https://github.com/nostr-protocol/nips/blob/master/59.md)