✨ add a "Minuten" unit to "Länge" form field

closes #5.

templates/download.html

@@ -54,10 +54,15 @@
 
         <div class="form-group required">
             {{ form.length.label(class_="form-control-label") }}
+            <div class='input-group'>
             {{ form.length(class_="form-control " + ("is-invalid" if form.length.errors else "")) }}
+            <div class="input-group-append">
+                <span class="input-group-text">Minuten</span>
+            </div>
             {% if form.length.errors %}
-                <div class="invalid-feedback">{{ form.length.errors|join("<br/>")|safe }}</div>
+            <div class="invalid-feedback">{{ form.length.errors|join("<br/>")|safe }}</div>
             {% endif %}
+            </div>
         </div>
 
         <div class="form-group required">