[Asset] Metadata Access if predicate

[nimdanor]

Version 1.0:
Access is defined in the activities.
If you whant that an exercise is only don in certain conditions you have to code it a programmable activity
Version 2.0. :
In version 2.0 it will be possible to change access rules in the metadata of the asset.
The predicate will be defined on success (or failure) of the current cours activities.