Add weekly cohort in addition to start date

This should allow us to do the 28 days later, and generally see retention on a week by week basis. 

Format is `yyyymmdd,week#`. Weeks are given a number starting from `1` for jan 1 in 2014. For each year week numbers increment by `100`. So jan 1 2015 is `101`, week 52 in 2015 will be `152`. The GA matchers for using this data are really flexible (regex!), so they should be easy to match either monthly or weekly, depending on our needs.