Activity streams

@voz Can you please review my code.

NOTES : 
- Libraries need to be deployed for the gadgets to work (in the folder .http://graasp.epfl.ch/gadget/libs/)
- DO NOT USE AddActivity gadget on graasp.epfl.ch before deploying the new activity system, it will not work and might corrupt the Activity table. (On react test everything will work fine)
- A working version of these gadgets is hosted on http://aubry.cholleton.free.fr/activity_streams/