Support animated emojis in Emoji's Display impl (serenity-rs#891)

src/model/guild/emoji.rs

@@ -221,9 +221,14 @@ impl Display for Emoji {
     /// Formats the emoji into a string that will cause Discord clients to
     /// render the emoji.
     ///
-    /// This is in the format of: `<:NAME:EMOJI_ID>`.
+    /// This is in the format of either `<:NAME:EMOJI_ID>` for normal emojis, 
+    /// or `<a:NAME:EMOJI_ID>` for animated emojis.
     fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
-        f.write_str("<:")?;
+        if self.animated {
+            f.write_str("<a:")?;
+        } else {
+            f.write_str("<:")?;
+        }
         f.write_str(&self.name)?;
         FmtWrite::write_char(f, ':')?;
         Display::fmt(&self.id, f)?;