providers/saml: big cleanup, simplify base processor

add New fields for
 - assertion_valid_not_before
 - assertion_valid_not_on_or_after
 - session_valid_not_on_or_after
allow flexible time durations for these fields
fall back to Provider's ACS if none is specified in AuthNRequest

passbook/providers/saml/migrations/0002_auto_20200214_1354.py

@@ -0,0 +1,61 @@
+# Generated by Django 2.2.9 on 2020-02-14 13:54
+
+from django.db import migrations, models
+
+import passbook.providers.saml.utils.time
+
+
+def migrate_valid_for(apps, schema_editor):
+    """Migrate from single number standing for minutes to 'minutes=3'"""
+    SAMLProvider = apps.get_model("passbook_providers_saml", "SAMLProvider")
+    db_alias = schema_editor.connection.alias
+    for provider in SAMLProvider.objects.using(db_alias).all():
+        provider.assertion_valid_not_on_or_after = (
+            f"minutes={provider.assertion_valid_for}"
+        )
+        provider.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("passbook_providers_saml", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="samlprovider",
+            name="assertion_valid_not_before",
+            field=models.TextField(
+                default="minutes=5",
+                help_text="Assertion valid not before current time - this value (Format: hours=1;minutes=2;seconds=3).",
+                validators=[
+                    passbook.providers.saml.utils.time.timedelta_string_validator
+                ],
+            ),
+        ),
+        migrations.AddField(
+            model_name="samlprovider",
+            name="assertion_valid_not_on_or_after",
+            field=models.TextField(
+                default="minutes=5",
+                help_text="Assertion not valid on or after current time + this value (Format: hours=1;minutes=2;seconds=3).",
+                validators=[
+                    passbook.providers.saml.utils.time.timedelta_string_validator
+                ],
+            ),
+        ),
+        migrations.RunPython(migrate_valid_for),
+        migrations.RemoveField(model_name="samlprovider", name="assertion_valid_for",),
+        migrations.AddField(
+            model_name="samlprovider",
+            name="session_valid_not_on_or_after",
+            field=models.TextField(
+                default="minutes=86400",
+                help_text="Session not valid on or after current time + this value (Format: hours=1;minutes=2;seconds=3).",
+                validators=[
+                    passbook.providers.saml.utils.time.timedelta_string_validator
+                ],
+            ),
+        ),
+    ]