Allow group orga to add a tracking script (analytics)

[loicknuchel]

It can be nice for orga to track the trafic in their public group pages.
An easy and powerful way can be to embed a user defined script on their page so they can use google analytics or whatever solution to gather their stats.
It can be risky to let users define a script, maybe we should just let them configure their analytics key but this imply we will have to support multiple analytics providers :(