feat(discord): add mention_only config for @-mention trigger (#529)

When mention_only is true, the bot only responds to messages that
@-mention the bot. Other messages in the guild are silently ignored.
Also strips the bot mention from content before processing.

Co-authored-by: Will Sarg <12886992+willsarg@users.noreply.github.com>

src/cron/scheduler.rs

@@ -245,6 +245,7 @@ async fn deliver_if_configured(config: &Config, job: &CronJob, output: &str) ->
                 dc.guild_id.clone(),
                 dc.allowed_users.clone(),
                 dc.listen_to_bots,
+                dc.mention_only,
             );
             channel.send(output, target).await?;
         }