auth: Make supported authentication backends a bitfield on realm.

This makes it possible to configure only certain authentication
methods to be enabled on a per-realm basis.

Note that the authentication_methods_dict function (which checks what
backends are supported on the realm) requires an in function import
due to a circular dependency.

zerver/migrations/0040_realm_authentication_methods.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import bitfield.models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0039_realmalias_drop_uniqueness'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='realm',
+            name='authentication_methods',
+            field=bitfield.models.BitField(['Google', 'Email', 'GitHub', 'LDAP', 'Dev', 'RemoteUser'], default=2147483647),
+        ),
+    ]