Remove std feature that got introduced in 1.36.0

std::convert::From<&std::string::String> for String was introduced in
1.36.0 and version below that version fail. Use `to_string()` instead to
make it build again.

ui/src/components/contacts.rs

@@ -127,7 +127,7 @@ impl ContactManager {
         self.form.push(("URL".into(), self.card.url().to_string()));
         self.form.push(("KEY".into(), self.card.key().to_string()));
         for (k, v) in self.card.extra_properties() {
-            self.form.push((k.into(), v.to_string()));
+            self.form.push((k.to_string(), v.to_string()));
         }
     }