Fix #379 -- Add logo to event organizers (#431)

* [WIP] Add logo to event organizers.

* Fix indentation issues.

* Refactor code

Refactor code

Refactor code

* Add new migration

* Take files into account for organizer sform (settings form)

* Fix grammer

* Make bootstrap form errors specific to each fieldset

* Display logo on organizer's page

* Fix PR issues

Fix PR issues

Fix PR issues

* Reorder imports

* Remove conflicting migration

* Fix rebase conflict

src/pretix/control/templates/pretixcontrol/base.html

@@ -65,7 +65,7 @@
                         <a href="#" class="dropdown-toggle" data-toggle="dropdown"><i class="fa fa-calendar"></i>
                             {{ request.event }} <span class="caret"></span></a>
                         <ul class="dropdown-menu" role="menu">
-                            <li><a href="{% url "control:events" %}">{% trans "Event overview" %}</a></li>
+                            <li><a href="{% url "control:events" %}">{% trans "Events overview" %}</a></li>
                             {% regroup request.user.events_cache by organizer as event_list %}
                             {% for g in event_list %}
                                 <li class="dropdown-header">{{ g.grouper }}</li>