Make navigation structure more approachable to new users (#1083)

* Move event selector to sidebar

* Unify navigation

* Fix confusing icons

src/pretix/control/templates/pretixcontrol/event/settings.html

@@ -2,6 +2,7 @@
 {% load i18n %}
 {% load bootstrap3 %}
 {% block inside %}
+    <h1>{% trans "General settings" %}</h1>
     <form action="" method="post" class="form-horizontal">
         {% csrf_token %}
         {% bootstrap_form_errors form %}