add envelope icon to invite friends default btn

app/assets/javascripts/discourse/templates/modal/invite.js.handlebars

@@ -18,7 +18,7 @@
   {{#if finished}}
     <button class='btn btn-primary' {{action closeModal}}>{{i18n close}}</button>
   {{else}}
-    <button class='btn btn-primary' {{bind-attr disabled="disabled"}} {{action createInvite}}>{{buttonTitle}}</button>
+    <button class='btn btn-primary' {{bind-attr disabled="disabled"}} {{action createInvite}}><i class="fa fa-envelope"></i>{{buttonTitle}}</button>
   {{/if}}
 
 </div>