Add ability to global query a user's wikis

Added an option that allows cohort owners to search across a wiki
user's accounts via CentralAuth and make a comprehensive cohort
out of all their accounts.

Dependencies:
https://gerrit.wikimedia.org/r/#/c/168600/
https://gerrit.wikimedia.org/r/#/c/167821/
https://gerrit.wikimedia.org/r/#/c/169018/

Change-Id: I57f9103eac662ca1a85627dacba829e780c795e5
Merge "Add ability to global query a user's wikis"