compose: Don't convert bot mentions to silent mentions.

This fixes an unintended consequence of the silent mention conversion
logic added in 4d1ade1f88, where bots
that looked for personal mentions would not process mentions in 1:1
DMs.

web/src/compose_validate.js

@@ -844,6 +844,13 @@ export function convert_mentions_to_silent_in_direct_messages(mention_text, full
         return mention_text;
     }
 
+    const user = people.get_user_by_id_assert_valid(user_id);
+    if (user.is_bot) {
+        // Since bots often process mentions as requests for them to
+        // do things, prefer non-silent mentions when DMing them.
+        return mention_text;
+    }
+
     const mention_str = people.get_mention_syntax(full_name, user_id, false);
     const silent_mention_str = people.get_mention_syntax(full_name, user_id, true);
     mention_text = mention_text.replace(mention_str, silent_mention_str);