Function send_message 

pub async fn send_message<T: Transport + ?Sized>(
    transport: &T,
    community: &Community,
    channel: &Channel,
    author: &Keys,
    content: &str,
    ms: u64,
) -> Result<Event, String>

Publish a Community message and retain its ephemeral key in the account DB so the sender can delete it later. Returns the published outer event.