Resolve "Replace django-easy-audit"

Closes #319 (closed)

• Replace backend code with reversions (@nik)
• Beautify template snippet (@hansegucker)
• Ensure that Alsijil makes use of revision in all relevant parts (AlekSIS-App-Alsijil#117 (closed))

aleksis/core/migrations/0005_timestamped_activity_notification.py

+# Generated by Django 3.1.3 on 2020-11-16 13:04
+
+from django.db import migrations, models
+import django.utils.timezone
+import model_utils.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0004_add_permissions_for_group_stats'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='activity',
+            name='created',
+            field=model_utils.fields.AutoCreatedField(default=django.utils.timezone.now, editable=False, verbose_name='created'),
+        ),
+        migrations.AddField(
+            model_name='activity',
+            name='modified',
+            field=model_utils.fields.AutoLastModifiedField(default=django.utils.timezone.now, editable=False, verbose_name='modified'),
+        ),
+        migrations.AddField(
+            model_name='notification',
+            name='created',
+            field=model_utils.fields.AutoCreatedField(default=django.utils.timezone.now, editable=False, verbose_name='created'),
+        ),
+        migrations.AddField(
+            model_name='notification',
+            name='modified',
+            field=model_utils.fields.AutoLastModifiedField(default=django.utils.timezone.now, editable=False, verbose_name='modified'),
+        ),
+    ]