Activities: Filter out objects from activity stream.

* Remove objects from the results that the authenticating user has no access to
* If above removal results in all activities being removed from the result, AccountService#getActivities will recursively repeat the find.
Log user actions

* Log a CREATE activity when a River, Bucket or Form is created.
* Log a FOLLOW activity when an account follows a River, Bucket or another Account
* Invites and Comments pending
Change GetActivityDTO.actionOnObj to java.lang.Object

* No every SwiftRiver object fits GetActivityDTO.Target
Log collaboration invites.

* Added RiverCollaboratorActivity + BucketCollaboratorActivity and
their creation when the respective collaborator is added.
Created specific bucket and river GetCollaboratorDTOs

* River and Bucket objects needed specifically for the activity stream.
Added getFollowedAccountsActivites

* Gets the activities of the accounts the authenticated user is
following.
Fix isOwner

* fix for NullPointerException the Account.collaboratingRiver is null