Add the quota to the external API for use with the mobile and desktop clients

GET /ocs/v1.php/cloud/users/$user 

```xml
<?xml version="1.0"?>
<ocs>
    <meta>
        <status>ok</status>
        <statuscode>100</statuscode>
        <message/>
    </meta>
    <data>
        <quota>
            <free>3677700096</free>
            <used>22849805</used>
            <total>3700549901</total>
            <relative>0.62</relative>
        </quota>
    </data>
</ocs>
```