Flock: added Advisory subscription support

see https://github.com/GigaOM/legacy-pro/issues/4525
All the elements in this form are manually prefixed with go-subscriptions.  This should be changed to use the pattern we typically use, where we use `get_field_name` and `get_field_id` functions to handle the prefixing.

Here is an imperfect example from `go-refried-phoenix`:

Function definitions (though I would make them `private`): 
https://github.com/GigaOM/gigaom-plugins/blob/master/go-refried-phoenix/components/class-go-refried-phoenix.php#L379

Use in a template (consider using `$this` instead of the singleton):
https://github.com/GigaOM/gigaom-plugins/blob/master/go-refried-phoenix/components/templates/analyst_connect_form.php#L166