- Have `Uniquify` take a block instead of a Proc/function. This is more
  idiomatic than passing around a function in Ruby.

- Block a user before moving their issues to the ghost user. This avoids a data
  race where an issue is created after the issues are migrated to the ghost user,
  and before the destroy takes place.

- No need to migrate issues (to the ghost user) in a transaction, because
  we're using `update_all`

- Other minor changes

- "Associated" issues are issues the user has created + issues that the
  user is assigned to.

- Issues that a user owns are transferred to a "Ghost User" (just a
  regular user with `state = 'ghost'` that is created when
  `User.ghost` is called).

- Issues that a user is assigned to are moved to the "Unassigned" state.

- Fix a spec failure in `profile_spec` — a spec was asserting that when a user
  is deleted, `User.count` decreases by 1. After this change, deleting a user
  creates (potentially) a ghost user, causing `User.count` not to change. The
  spec has been updated to look for the relevant user in the assertion.

Enable (and disable) more RuboCops

See merge request !9432

Fix reprocessing skipped jobs when retrying pipeline

See merge request !9436

This reverts commit e00fb2bdc2090e9cabeb1eb35a2672a882cc96e9.

# Conflicts:
#	.rubocop.yml
#	.rubocop_todo.yml
#	lib/gitlab/ci/config/entry/global.rb
#	lib/gitlab/ci/config/entry/jobs.rb
#	spec/lib/gitlab/ci/config/entry/factory_spec.rb
#	spec/lib/gitlab/ci/config/entry/global_spec.rb
#	spec/lib/gitlab/ci/config/entry/job_spec.rb
#	spec/lib/gitlab/ci/status/build/factory_spec.rb
#	spec/lib/gitlab/incoming_email_spec.rb

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.