GUAC-1260 Allow extensions to override guacamole icons.

guacamole/src/main/webapp/app/login/templates/login.html

@@ -31,7 +31,7 @@
             <form class="login-form" ng-submit="login()">
 
                 <!-- Guacamole version -->
-                <img class="logo" src="images/guac-tricolor.png" alt=""/>
+                <div class="logo"></div>
                 <div class="version">{{'APP.NAME' | translate}}</div>
 
                 <!-- Login message/instructions -->