UI: Improve panel collapse visibility (Z#23132549) (#3709)

* Move collapse indicator in panels before label

* only hide collapse indicator in panel titles

* remove unneeded collapse-indicators in pretix-control

* remove unneeded collapse-indicators in presale

* move collapse-indicator to left in variants-toggle-button

* remove chevron and use default collapse-indicator in control-variants

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

@@ -5,7 +5,6 @@
     <summary class="panel-heading">
         <h4 class="panel-title">
             <strong>{% trans title %}</strong>
-            <i class="fa fa-angle-down collapse-indicator"></i>
         </h4>
     </summary>
     <div id="{{ pid }}">