Track when user was last active

fixes #10

bookwyrm/migrations/0063_user_last_active_date.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2020-11-01 17:05
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('bookwyrm', '0062_auto_20201031_1936'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='user',
+            name='last_active_date',
+            field=models.DateTimeField(auto_now=True),
+        ),
+    ]