assets: Add "bullhorn.svg" icon for wildcard mentions

This is the icon Web uses; which is a Font Awesome 4.7.0 icon
(fa-bullhorn). This icon is changed in the newer versions. I couldn't
find its SVG from their website nor from their GitHub repository, but
from another repository:
  https://github.com/Rush/Font-Awesome-SVG-PNG/blob/master/black/svg/bullhorn.svg

lib/widgets/icons.dart

@@ -36,50 +36,53 @@ abstract final class ZulipIcons {
   /// The Zulip custom icon "bot".
   static const IconData bot = IconData(0xf104, fontFamily: "Zulip Icons");
 
+  /// The Zulip custom icon "bullhorn".
+  static const IconData bullhorn = IconData(0xf105, fontFamily: "Zulip Icons");
+
   /// The Zulip custom icon "chevron_right".
-  static const IconData chevron_right = IconData(0xf105, fontFamily: "Zulip Icons");
+  static const IconData chevron_right = IconData(0xf106, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "clock".
-  static const IconData clock = IconData(0xf106, fontFamily: "Zulip Icons");
+  static const IconData clock = IconData(0xf107, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "edited".
-  static const IconData edited = IconData(0xf107, fontFamily: "Zulip Icons");
+  static const IconData edited = IconData(0xf108, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "globe".
-  static const IconData globe = IconData(0xf108, fontFamily: "Zulip Icons");
+  static const IconData globe = IconData(0xf109, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "group_dm".
-  static const IconData group_dm = IconData(0xf109, fontFamily: "Zulip Icons");
+  static const IconData group_dm = IconData(0xf10a, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "hash_sign".
-  static const IconData hash_sign = IconData(0xf10a, fontFamily: "Zulip Icons");
+  static const IconData hash_sign = IconData(0xf10b, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "language".
-  static const IconData language = IconData(0xf10b, fontFamily: "Zulip Icons");
+  static const IconData language = IconData(0xf10c, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "lock".
-  static const IconData lock = IconData(0xf10c, fontFamily: "Zulip Icons");
+  static const IconData lock = IconData(0xf10d, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "message_moved".
-  static const IconData message_moved = IconData(0xf10d, fontFamily: "Zulip Icons");
+  static const IconData message_moved = IconData(0xf10e, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "mute".
-  static const IconData mute = IconData(0xf10e, fontFamily: "Zulip Icons");
+  static const IconData mute = IconData(0xf10f, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "read_receipts".
-  static const IconData read_receipts = IconData(0xf10f, fontFamily: "Zulip Icons");
+  static const IconData read_receipts = IconData(0xf110, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "star_filled".
-  static const IconData star_filled = IconData(0xf110, fontFamily: "Zulip Icons");
+  static const IconData star_filled = IconData(0xf111, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "topic".
-  static const IconData topic = IconData(0xf111, fontFamily: "Zulip Icons");
+  static const IconData topic = IconData(0xf112, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "unmute".
-  static const IconData unmute = IconData(0xf112, fontFamily: "Zulip Icons");
+  static const IconData unmute = IconData(0xf113, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "user".
-  static const IconData user = IconData(0xf113, fontFamily: "Zulip Icons");
+  static const IconData user = IconData(0xf114, fontFamily: "Zulip Icons");
 
   // END GENERATED ICON DATA
 }