feat: out-of-box experience (#2)

Reviewed-on: https://git.reya.su/reya/signed/pulls/2

crates/utils/src/pubkey.rs

@@ -0,0 +1,7 @@
+use nostr::prelude::*;
+
+/// Shorten a [`PublicKey`] to `npub1abc...wxyz` form.
+pub fn shorten_pubkey(public_key: PublicKey, len: usize) -> String {
+    let npub = public_key.to_bech32().unwrap();
+    format!("{}...{}", &npub[..(len + 5)], &npub[npub.len() - len..])
+}