make sure styles and scripts are only loaded once

refs https://github.com/owncloud/core/issues/12743

- [x] make sure js and css are added only once
- [x] loading of md5, avatar and placeholder only if logged in
- [x] load showpassword only if needed

Combine them:

```
script('core', [
    'visitortimezone',
    'lostpassword'
]);