Fixed #19177 -- Better layout for admin login form field labels.

Thanks goes to void for the comprehensive report, to Pedro Mourelle for
the fix and to Bryan Veloso, Łukasz Rekucki and Claude Paroz for the
review.

This fix includes the following changes:

* Modified layout to top-aligned labels.
* Added 'air' for inputs (the padding was really needed there.). It
  adjust good to zoom in/out.
* Removed float attributes from label and inputs in login form.
* Removed useless text-align property.

django/contrib/admin/static/admin/css/login.css

@@ -29,17 +29,20 @@ body.login {
 }
 
 .login .form-row label {
-    float: left;
-    width: 9em;
     padding-right: 0.5em;
     line-height: 2em;
-    text-align: right;
     font-size: 1em;
+    clear: both;
     color: #333;
 }
 
 .login .form-row #id_username, .login .form-row #id_password {
-    width: 14em;
+    clear: both;
+    padding: 6px;
+    width: 100%;
+    -webkit-box-sizing: border-box;
+       -moz-box-sizing: border-box;
+            box-sizing: border-box;
 }
 
 .login span.help {