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/channels/mod.rs

@@ -620,6 +620,7 @@ pub async fn doctor_channels(config: Config) -> Result<()> {
                 dc.guild_id.clone(),
                 dc.allowed_users.clone(),
                 dc.listen_to_bots,
+                dc.mention_only,
             )),
         ));
     }
@@ -906,6 +907,7 @@ pub async fn start_channels(config: Config) -> Result<()> {
             dc.guild_id.clone(),
             dc.allowed_users.clone(),
             dc.listen_to_bots,
+            dc.mention_only,
         )));
     }