People activity feed

[ferblape]

Follow up of #223

This wasn't implemented in #223 and we need to think a bit in the implementation.

@danguita thoughts:

It would be needed to attach all noticeable Person-related events to be able to render the Person Activity feed. An improvement could be to do so at FormObject level instead of Controller level, as we do in the User Subscription feature through the Trackable extension. We are now building an empty array of Activities just to have a placeholder to work with.