Generate a token and use it to bucket users

* Create the mw.gettingStarted.user object, which can be used to get a
  user's token and the bucket that they are in (getToken and getBucket
  respectively)
* Make mw.gettingStarted.user.getToken transparently persist the token
  to a cookie for 90 days

Change-Id: I4c35a2bece411e7f0b10ce9a22f2aa592b77c388
(cherry picked from commit 533c7f3b5821f6637f349dd0fd8ecd6a0de660d0)