Squash database migrations up to v18.11.0-ee

HOUSEKEEPER_GITLAB_API_TOKENrequested to merge
squash-migrations-18-11 into master

What does this MR do and why?

This MR removes database migrations up to origin/18-11-stable-ee and squashes them into db/init_structure.sql and removes associated specs and rubocop todos.

The changes were mainly created by running bundle exec rake gitlab:db:squash[origin/18-11-stable-ee].

This change was generated by gitlab-housekeeper in CI using the Keeps::SquashMigrations keep.

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Edited by 🤖 GitLab Bot 🤖

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