global: make a better separation between request-related and "business logic" code

[slint]

The various client handlers contain business logic related to how to e.g. disconnect a user from an OAuth service which shouldn't depend on the request context. Example from invenio_oauthclient.contrib.orcid.disconnect_handler:

...
def disconnect_handler(remote, *args, **kwargs):
    """Handle unlinking of remote account.
    :param remote: The remote application.
    """
    # ...this is request-related...
    if not current_user.is_authenticated: 
        return current_app.login_manager.unauthorized()

    # -- START --
    # This part should be in a different function, since it only depends on a User ID
    account = RemoteAccount.get(user_id=current_user.get_id(),
                                client_id=remote.consumer_key)
    orcid = account.extra_data.get('orcid')

    if orcid:
        oauth_unlink_external_id({'id': orcid, 'method': 'orcid'})
    if account:
        with db.session.begin_nested():
            account.delete()
    # -- END --

    # ...this is request-related...
    return redirect(url_for('invenio_oauthclient_settings.index'))
...