Fix font size for collapse button

app/assets/stylesheets/generic/nav_sidebar.scss

@@ -157,6 +157,7 @@
   top: 46px;
   padding: 5px 13px 3px 13px;
   left: 197px;
+  font-size: 13px;
   background: #EEE;
   color: black;
   border: 1px solid rgba(0,0,0,0.035);