Start deprecating IE11 support in backend

src/pretix/control/context.py

@@ -155,6 +155,8 @@ def _default_context(request):
         if not gs.settings.update_check_ack and 'runserver' not in sys.argv:
             ctx['warning_update_check_active'] = True
 
+    ctx['ie_deprecation_warning'] = 'MSIE' in request.headers['User-Agent'] or 'Trident/' in request.headers['User-Agent']
+
     if request.user.is_authenticated:
         ctx['staff_session'] = request.user.has_active_staff_session(request.session.session_key)
         ctx['staff_need_to_explain'] = (

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

@@ -318,6 +318,23 @@
                     </div>
                 </div>
             </nav>
+            {% if ie_deprecation_warning %}
+                <div class="old-browser-warning">
+                    <span class="fa fa-internet-explorer"></span>
+                    {% blocktrans trimmed %}
+                        We've detected that you are using <strong>Microsoft Internet Explorer</strong>.
+                    {% endblocktrans %}
+                    {% blocktrans trimmed %}
+                        Internet Explorer is an old browser that does not support lots of recent web-based
+                        technologies. While some features might already not work properly, we plan on no longer
+                        supporting Internet Explorer in our administrative backend in the next months.
+                    {% endblocktrans %}
+                    {% blocktrans trimmed %}
+                        We kindly ask you to move to one of our supported browsers, such as Microsoft Edge,
+                        Mozilla Firefox, Google Chrome, or Safari.
+                    {% endblocktrans %}
+                </div>
+            {% endif %}
             {% if staff_need_to_explain %}
                 <div class="impersonate-warning">
                     <span class="fa fa-id-card"></span>