Public api activity [WIP]

:construction: 

requires https://github.com/owncloud/core/pull/9412
the subjectparams is an array.
If this function returns an integer, the subjects are grouped by the parameter.

Example:
activities:
* subject: file_created, subjectparams: [file1, username1]
* subject: file_created, subjectparams: [file2, username1]
* subject: file_created, subjectparams: [file2, username2]

function returns 0 =>
* username1 created file1 and file2
* username2 created file2

function returns 1 => 
* username1 created file1
* username1 and username2 created file2