Change the token login input field to password (#2642)

This prevents the browsers from caching the value.

src/app/frontend/login/token.html

@@ -19,6 +19,7 @@ limitations under the License.
     <label>[[Enter token|'Token' field label shown on login page for token auth]]</label>
     <input ng-model="$ctrl.token"
            ng-change="$ctrl.onTokenUpdate()"
-           required>
+           required
+           type="password">
   </md-input-container>
 </div>