Log user ID; 'pageViewSessionId' to alphanum string

By default, EventLogging uses INT columns for integers, which pageViewSessionId
overflows. I changed it to a STRING type in the schema and am now generating it
using 'mw.user.generateRandomSessionId()'. I also introduced a 'userId'
property, per bug 49585.

Bug: 49585
Bug: 49586
Change-Id: Iddb9eb9c64b55b25445ddf9e474d312b685978a6