Discourage long event short forms

src/pretix/control/templates/pretixcontrol/events/create_basics.html

@@ -27,6 +27,11 @@
                         references as an abbreviation to reference this event.
                     {% endblocktrans %}
                 </div>
+                <div class="slug-length alert alert-warning helper-display-none-soft">
+                    {% blocktrans trimmed %}
+                        We strongly recommend against using short forms of more then 16 characters.
+                    {% endblocktrans %}
+                </div>
             </div>
         </div>
         {% bootstrap_field form.date_from layout="control" %}

src/pretix/static/pretixcontrol/js/ui/main.js

@@ -358,6 +358,10 @@ var form_handlers = function (el) {
     el.find(".simple-subevent-choice").change(function () {
         $(this).closest("form").submit();
     });
+
+    el.find("input[name=basics-slug]").bind("keyup keydown change", function () {
+        $(this).closest(".form-group").find(".slug-length").toggle($(this).val().length > 16);
+    });
 };
 
 $(function () {

src/pretix/static/pretixcontrol/scss/main.scss

@@ -139,6 +139,9 @@ p.bigger {
 .helper-display-inline {
     display: inline !important;
 }
+.helper-display-none-soft {
+    display: none;
+}
 .helper-display-none {
     display: none !important;
 }