Add ability to remove cohorts from database.

Owners can remove their cohorts from the database. A warning will show if there
are other users sharing the cohort. When deleted, viewers records will be
deleted from their personal list. If a viewer deletes a cohort, it will only
delete the cohort from their personal list, not from the cohort database.

Change-Id: I431cad933d5b888f10fe5533b75e7b1776649ab3