Enable collaboration by default on activities

When we implemented read max_participants from activity.info [1]
we changed the default value of max_participants, from 0 to 1.
Before, was responsability of the activities set max_participants == 1,
when the activity not implemented collaboration.
This change make the default backwards compatible.

[1] d0cca91fe8510e11de8f332bf659789daf9c0d8d