[IMP] scss: make alert full width of parent

Before this commit, the alerts would all have different widths according
to their content, some stretching and getting pushed under the
"on this page" menu. This way all alerts are aligned.

Part-of: odoo/documentation#1668

extensions/odoo_theme/static/style.scss

@@ -856,6 +856,7 @@ header.o_main_header {
             display: inline-block;
             border-radius: 0;
             border-width: 0 0 0 3px;
+            width: 100%;
             @include font-size($font-size-secondary);
 
             code {