Add new /cloud/capabilities route

Produces the following result:
```xml
<ocs>
<meta>
<status>ok</status>
<statuscode>100</statuscode>
<message/>
</meta>
<data>
<version>4.91.2</version>
<versionstring>5.0 pre alpha</versionstring>
<edition/>
<bugfilechunking>true</bugfilechunking>
<encryption>false</encryption>
<versioning>false</versioning>
<undelete>true</undelete>
<installedapps>
<element>files</element>
<element>user_migrate</element>
<element>admin_migrate</element>
<element>files_texteditor</element>
<element>provisioning_api</element>
</installedapps>
</data>
</ocs>
```

@karlitschek  would you like installed apps or _all_ apps?