Fixed #149: Form backup should work for all forms.

Resolves #149

To support more than just the issue node/comment form, the form backup storage now uses a `.<form_id>` suffix.

The old `form.backup` storage key unfortunately cannot be cleaned up, as we do not have a update system yet (which would likely work similar to `hook_update_N()`, but we would use unix timestamps instead of arbitrary sequential numbers).