Fix specificity of login button CSS, to restore large button style.

Thanks to @sebworks for the bug report and investigation!

CONTRIBUTORS.rst

@@ -156,6 +156,7 @@ Contributors
 * Chris Darko
 * Aymeric Augustin
 * Adriaan Tijsseling
+* sebworks
 
 Translators
 ===========

wagtail/wagtailadmin/static_src/wagtailadmin/scss/layouts/login.scss

@@ -54,7 +54,7 @@ form {
         color: $color-white;
     }
 
-    button {
+    .button {
         font-size: 1.5em;
         padding: 1.1em 2.4em;
         height: 3.5em;