Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

1. Extract out into several methods.
2. Add more comments describing the data and the shape of the data.

Still need final links all over the place.

This table shows the percentage of users who registered in the last
twelve months, who last signed in during or later than each of those
twelve months, by month.

It is only enabled when the usage ping is enabled, and the page also
shows pretty-printed usage ping data.

The cohorts table is generated in Ruby from some basic SQL queries,
because performing the gap-filling and running sums needed in both MySQL
and Postgres is painful.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

This new class uses a Redis Hash instead of a Sorted Set.
Signed-off-by: Rémy Coutable <remy@rymai.me>

CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/962

Refactored specs and added a post deployment migration to remove the activity users table.

It uses a user activity table instead of a column in users.
Tested with mySQL and postgreSQL

CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1521