Use hook_og to allow subscribing and unsubscribing via OG API

Mailman Groups should implement hook_og from the OG API instead of form_alter.

This has two main benefits:
1) The code is more reliable and easier to understand/maintain
2) The implementation of hook_og is unlikely to change, whereas the form components from could change without notice
3) It provides a reliable way of handling unsubscribe requests

I've attached a patch that implements this, and deletes the _mailman_groups_alter_og_confirm_subscribe function which is no longer needed. This hasn't been extensively tested, but has been running okay for me in a live site for the past few days.