Fix #630 -- manual check-in of attendees (#642)

* [WIP] manual check-in of attendees

This enables manual check-in of attendees.  The post-code was heavily
copied from the APIRedeemView of the pretixdroid, thus so far this seems
to be working, but I have a few questions:

The checkin-Objects generated by the pretixdroid-app have a nonce.
Should the checkin object generated here have a nonce, too?

Should the result of the check-in be noted in any other way than by the
change of the status?

* addressed review comments

* implement unit test for manual checkin

* fix style-issues

* Slight layout change

* Log who did the manual check-in

* Improve unit test to check the result of the action

src/pretix/control/logdisplay.py

@@ -1,8 +1,11 @@
 import json
 from decimal import Decimal
 
+import dateutil.parser
+import pytz
 from django.dispatch import receiver
 from django.utils import formats
+from django.utils.formats import date_format
 from django.utils.translation import pgettext_lazy, ugettext_lazy as _
 from i18nfield.strings import LazyI18nString
 
@@ -200,6 +203,21 @@ def pretixcontrol_logentry_display(sender: Event, logentry: LogEntry, **kwargs):
     if logentry.action_type.startswith('pretix.event.tickets.provider.'):
         return _('The settings of a ticket output provider have been changed.')
 
+    if logentry.action_type == 'pretix.control.views.checkin':
+        dt = dateutil.parser.parse(data.get('datetime'))
+        tz = pytz.timezone(sender.settings.timezone)
+        dt_formatted = date_format(dt.astimezone(tz), "SHORT_DATETIME_FORMAT")
+
+        if data.get('first'):
+            return _('Position #{posid} has been checked in manually at {datetime}.').format(
+                posid=data.get('positionid'),
+                datetime=dt_formatted
+            )
+        return _('Position #{posid} has been checked in again at {datetime}.').format(
+            posid=data.get('positionid'),
+            datetime=dt_formatted
+        )
+
     if logentry.action_type == 'pretix.team.member.added':
         return _('{user} has been added to the team.').format(user=data.get('email'))