Fix bug on visiting a segment page in the admin

Wagtail's InlinePanel's relies on
django.db.models.options.Options.get_field -
bcf6b6da77/wagtail/admin/edit_handlers.py (L688)
which seems to have a bug of using related_query_name rather than
related_name naming and therefore trying to edit segment always
fails - https://code.djangoproject.com/ticket/29458. The bug has not
yet been confirmed by others but it's impossible to edit segments.

Therefore this change deletes related_query_name from relations from
rules to the Segment model.

src/wagtail_personalisation/models.py

@@ -121,7 +121,7 @@ class Segment(ClusterableModel):
             ], heading="Segment"),
             MultiFieldPanel([
                 InlinePanel(
-                    "{}_related".format(rule_model._meta.db_table),
+                    "{}s".format(rule_model._meta.db_table),
                     label='{}{}'.format(
                         rule_model._meta.verbose_name,
                         ' ({})'.format(_('Static compatible')) if rule_model.static else ''