Fix cloud/activity API page

The route still exists but the method for it was dropped previously. Also the page was only available for admins.

This PR restores the old behaviour again:
ocs/v1.php/cloud/activity:
Parameters: `start`, `count`
```
<?xml version="1.0"?>
<ocs>
 <meta>
  <status>ok</status>
  <statuscode>100</statuscode>
  <message/>
 </meta>
 <data>
  <element>
   <id>487</id>
   <subject>You deleted n-t/announce_read.gif</subject>
   <message></message>
   <file>/n-t/announce_read.gif</file>
   <link>http://www.owncloud.local/master/index.php/apps/files?dir=%2Fn-t</link>
   <date>2014-06-11T10:56:15+00:00</date>
  </element>
 </data>
</ocs>
```

@karlitschek @MTRichards