Centre login form across all devices. (#49)

* Centre login window on tablet, phones and desktop.

bikeshop_project/registration/templates/login.html

@@ -35,7 +35,7 @@
 
 
 {% block content %}
-<div class="demo-graphs mdl-shadow--2dp mdl-color--white mdl-cell mdl-cell--8-col mdl-shadow--2dp mdl-cell--2-offset">
+<div class="demo-graphs mdl-shadow--2dp mdl-color--white mdl-cell mdl-cell--12-col mdl-cell--8-col-desktop mdl-shadow--2dp mdl-cell--2-offset-desktop">
     <h2 class="mdl-card__title-text">Login</h2>
     <form method="post">
         {% if form.non_field_errors %}