Remove all instances of theme_id; require downtime

config/gitlab.yml.example

db/migrate/20170213172852_remove_theme_id_from_users.rb

db/schema.rb

doc/api/keys.md

doc/api/session.md

doc/api/users.md

lib/api/entities.rb

spec/controllers/profiles/preferences_controller_spec.rb

spec/fixtures/api/schemas/user/login.json

spec/fixtures/api/schemas/user/public.json

spec/models/user_spec.rb

spec/support/gitlab_stubs/session.json

spec/support/gitlab_stubs/user.json