[Bug] Mentions should use NIP-27

Currently mentions are not interoperable with other Nostr clients. It's fine to type @username in the text box, but after hitting submit the event it generates should include a [NIP-27](https://github.com/nostr-protocol/nips/blob/master/27.md) Nostr URI.

It's not just a compatibility issue. People on Nostr can change their username. This would break existing mentions. Plus there's not really any concept of a "username". The name can contain spaces. The correct solution is NIP-27.

Start by tracing how mentions move from the text box through submission into the generated Nostr event. Compare the resulting mention representation with NIP-27 and verify that the submitted event includes a NIP-27 Nostr URI so mentions remain interoperable and stable when names change.

Written by the indexing model from the issue text.