In issue 1, the sequential scan was taking 250+ ms.

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

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

CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/557

I started out as I missed the foreign key on ci_triggers and projects.
However, it seemed Yorick caught this already on gitlab-org/gitlab-ce!9388.

Still I found a missing foreign key, between the trigger requests and
triggers. This commit adds it, and removes the depend: :destroy on the
Rails model Ci::Triggers