Use FontAwesome icon for the sheet app

www/common/application_config_internal.js

@@ -138,7 +138,7 @@ define(function() {
         kanban: 'cptools-kanban',
         doc: 'fa-file-word-o',
         presentation: 'fa-file-powerpoint-o',
-        sheet: 'cptools-sheet',
+        sheet: 'fa-file-excel-o',
         drive: 'fa-hdd-o',
         teams: 'fa-users',
     };

www/form/inner.js

@@ -2640,7 +2640,7 @@ define([
         // Export in "sheet"
         Messages.form_exportSheet = "Export to spreadsheet"; // XXX
         var export2Button = h('button.btn.btn-primary', [
-            h('i.cptools.cptools-sheet'),
+            h('i.cptools.fa-file-excel-o'),
             Messages.form_exportSheet
         ]);
         $(export2Button).appendTo($controls);