Make it possible to block OAuth signin to accounts

This adds a checkbox to each user account letting the user block the use
of OAuth to this account, forcing the use of internal authentication.

pgweb/core/migrations/0002_block_oauth.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.17 on 2019-02-10 13:21
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='userprofile',
+            name='block_oauth',
+            field=models.BooleanField(default=False, help_text='Disallow login to this account using OAuth providers like Google or Microsoft.', verbose_name='Block OAuth login'),
+        ),
+    ]