feat(preferences) - create a way to register per resource preferences and fetch them

Should supersede https://github.com/superdesk/superdesk-server/pull/272
i would rather use some explicit way of declaring it, like:
```
superdesk.register_user_option('email.notification', {
    'type': 'bool',
    'default': True,
    'label': 'Send notifications via email',
    'category': 'notifications',
})
```
and usage could be via ```superdesk.user_option('email.notification')```