Introduce hidden plan

• Closes #60 (closed)
• Add boolean flag to event
• Hide plan for non-staff users when flag is set
• Update translations

AKModel/migrations/0035_event_plan_hidden.py

+# Generated by Django 3.0.6 on 2020-05-19 20:08
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('AKModel', '0034_room_rename_field'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='event',
+            name='plan_hidden',
+            field=models.BooleanField(default=True, help_text='Hides plan for non-staff users', verbose_name='Plan Hidden'),
+        ),
+    ]