Fix dropdown divider in header

Renamed .divider to .dropdown-divider.

see https://getbootstrap.com/docs/4.0/migration/#dropdowns

Author: vanam

pug/partials/header.pug

@@ -63,7 +63,7 @@ header.app-header.navbar
             i.fa.fa-file
             |  Projects
             span.badge.badge-primary 42
-          .divider
+          .dropdown-divider
           a.dropdown-item(href="#")
             i.fa.fa-shield
             |  Lock Account