realm_creation_form: Capture import_from if realm import enabled.

We store user's preference for `import_from` to be acted upon in
later commits.

zerver/models/prereg_users.py

@@ -50,6 +50,10 @@ class PreregistrationRealm(models.Model):
         UserProfile, null=True, related_name="+", on_delete=models.SET_NULL
     )
 
+    IMPORT_FROM_CHOICES = [
+        ("none", "Don't import"),
+        ("slack", "Import from Slack"),
+    ]
     data_import_metadata = models.JSONField(default=dict, encoder=DjangoJSONEncoder)