Add stub modify_state to make test_interface happy

GroupState is still going to change, so no point implementing this right
now. Instead, implement a stub so test_interface tells me the next thing
to implement.
test_delete_webhook_happy_case
test_delete_webhook_for_nonexistant_scaling_group
test_delete_webhook_for_nonexistant_policy
test_delete_webhook_for_nonexistant_webhook
Factor out some logic
Rework that callback a little
Argh, nope

I can't do this the nice way because the result_proxies for delete and
get queries behave very differently.
Make some of the tests pass
ON DELETE CASCADE for webhook metadata
Make all the tests pass
test_list_webhooks_for_nonexistent_policy
Fix test name
test_list_webhooks_happy_case
Fix a typo in the test
Make a test pass
Make another test pass
Implement stub delete_group
Implement stub view_state
test_update_config_for_nonexistent_group