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/widget.html

@@ -5,7 +5,7 @@
 {% load eventurl %}
 {% load eventsignal %}
 {% block inside %}
-    <legend>{% trans "Widget" %}</legend>
+    <h1>{% trans "Widget" %}</h1>
     <p>
         {% blocktrans trimmed %}
             The pretix widget is a way to embed your ticket shop into your event website. This way, your visitors can