Memberships: Prefer valid_from over event date for .is_valid() (#4003)

* Memberships: Prefer valid_from over event date for .is_valid()

* Fix tests

* Add parameter description

* Use reasonable default for requested_valid_from if membership starts in the future

* Set datetimepicker viewDate to closest allowed date

* Keep current value on going back to QuestionsStep

* Fix min_date/max_date in SplitDateTimePickerWidget

* Remove unused import

* Update src/pretix/base/models/memberships.py

Co-authored-by: Mira <weller@rami.io>

* Respect variations

---------

Co-authored-by: Mira Weller <weller@rami.io>

src/pretix/control/templates/pretixcontrol/item/index.html

@@ -267,6 +267,7 @@
                         {% bootstrap_field form.show_quota_left layout="control" %}
                         {% for f in plugin_forms %}
                             {% if not f.is_layouts and not f.title %}
+                                <hr />
                                 {% if f.template and not "template" in f.fields %}
                                     {% include f.template with form=f %}
                                 {% else %}