Various CSS improvements (#514)

* Replace flexbox grid with CSS grid

* Update new and edit forms

* Update settings views

* Update auth views

* Fix margin in menu

* Remove unused Spectre modules

* Simplify navbar

* Reuse CSS variables

* Fix grid gap on small screen sizes

* Simplify grid system

* Improve section headers

* Restructure SASS files

* Cleanup base styles

* Update test

bookmarks/templates/django_registration/registration_complete.html

@@ -4,13 +4,5 @@
 {% block title %}Registration complete{% endblock %}
 
 {% block content %}
-
-    <div class="auth-page">
-        <div class="columns">
-            <section class="content-area column col-12">
-                <p>Registration complete. You can now use the application.</p>
-            </section>
-        </div>
-    </div>
-
+  <p>Registration complete. You can now use the application.</p>
 {% endblock %}