[AIRFLOW-3302] Small CSS fixes (apache#4140)

* Don't highlight logout button when viewing Log tab of a task
* Align Airflow logo to the center of the login page

airflow/www/templates/admin/master.html

@@ -89,7 +89,7 @@
         <ul class="nav navbar-nav navbar-right">
             <li><a id="clock"></a></li>
             {% if current_user.is_authenticated %}
-              <li><a href="{{ url_for('airflow.logout') }}"><span data-toggle="tooltip" data-placement="left" title="Logout" class="glyphicon glyphicon-log-out"></span></a></li>
+              <li class="never_active"><a href="{{ url_for('airflow.logout') }}"><span data-toggle="tooltip" data-placement="left" title="Logout" class="glyphicon glyphicon-log-out"></span></a></li>
             {% endif %}
         </ul>
           <ul class="nav navbar-nav navbar-right">

airflow/www/templates/airflow/login.html

@@ -1,13 +1,13 @@
-{# 
+{#
   Licensed to the Apache Software Foundation (ASF) under one or more
   contributor license agreements.  See the NOTICE file distributed with
   this work for additional information regarding copyright ownership.
   The ASF licenses this file to You under the Apache License, Version 2.0
   (the "License"); you may not use this file except in compliance with
   the License.  You may obtain a copy of the License at
-  
+
     http://www.apache.org/licenses/LICENSE-2.0
-  
+
   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
@@ -25,7 +25,7 @@
     <div class="row">
         <div class="col-sm-6 col-md-4 col-md-offset-4">
             <h1 class="text-center login-title">Sign in to Airflow</h1>
-            <div class="account-wall">
+            <div class="text-center account-wall">
                 <img src="{{ url_for("static", filename="pin_100.png") }}" />
                 <form class="form-signin" method="post" action="{{ url_for('airflow.login') }}">
                     <input type="text" class="form-control" placeholder="Username" name="username" required autofocus>
@@ -43,4 +43,4 @@ <h1 class="text-center login-title">Sign in to Airflow</h1>
     </div>
 </div>
 
-{% endblock %}
+{% endblock %}