moving the 'set_password' method into the adapter

Hi,

I've moved the set_password method from `account/forms.py` to `account/adapter.py`, so now from the forms it can easily be done by `get_adapter().set_password(user, password)`.

I think this makes sense because the other user modifying methods are already using the adapter, so it's only consistent (f.e. `save_user()`).

Our use case:
We are using django-allauth in our public cloud web interface. But beside keeping the users in the local database which is used by the web interface, we also need to sync those users to other third party systems that run the cloud. So to implement the forgot password / change passsword procedures, currently we can only put our hooks that sync to the third party systems into the model, which i think is not optimal. The logic to sync the creation of users to the third party systems is currently already in the adapter, so i would like to also have the logic to sync the password reset in the adapter.

Cheers