ReplyMentions: to.length --> to.size, fix bug replying to self

app/soapbox/features/compose/components/reply_mentions.js

@@ -23,7 +23,7 @@ class ReplyMentions extends ImmutablePureComponent {
   render() {
     const { explicitAddressing, to, isReply } = this.props;
 
-    if (!explicitAddressing || !isReply || !to || to.length === 0) {
+    if (!explicitAddressing || !isReply || !to || to.size === 0) {
       return null;
     }
 
@@ -41,4 +41,4 @@ class ReplyMentions extends ImmutablePureComponent {
     );
   }
 
-}
+}