Unify concept of absolute_id and remote_id

fedireads/migrations/0041_user_remote_id.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.3 on 2020-05-13 02:23
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('fedireads', '0040_auto_20200513_0153'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='user',
+            name='remote_id',
+            field=models.CharField(max_length=255, null=True, unique=True),
+        ),
+    ]