Add topbar navigation to all pages and make logo clickable (Issues #14, #13)

- Create reusable _nav.html navigation include
- Add topbar to all settings pages (settings, profile, communities, filters, experience)
- Add topbar to all admin pages (admin, polling, polling_logs, setup)
- Replace hardcoded nav in dashboard with include
- Wrap logo in link to index page (fixes clicking logo to go home)

Fixes #14, #13

templates/settings_communities.html

@@ -235,6 +235,7 @@
 {% endblock %}
 
 {% block content %}
+{% include '_nav.html' %}
 <div class="settings-container">
     <div class="settings-header">
         <h1>Community Settings</h1>