Migrate `ghost` logic to user_type column

Summary

This is part of #219001 (closed)

We need to migrate "ghost" logic to use user_type

Improvements

Simpler schema, simpler indexes, simpler code

Risks

Ghosts related functionality can be broken if something goes wrong.

Involved components

• app/models/user.rb
• db/schema.rb

Optional: Intended side effects

Ghost users can't be a ghost AND an alert\support bot at the same time. At GitLab.com ghost user is not a bot, so we are good here.

/cc @gitlab-org/maintainers/database WDYT?