GUAC-1260 Allow extensions to override guacamole icons.

guacamole/src/main/webapp/app/login/styles/dialog.css

@@ -83,8 +83,11 @@
 
 .login-ui .login-dialog .logo {
     display: block;
-    max-width: 3em;
     margin: 0.5em auto;
+    width: 3em;
+    height: 3em;
+    background-size: 3em;
+    background-image: url("images/guac-tricolor.png");
 }
 
 .login-ui.continuation div.login-dialog {

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 -->