Auto-generate stable fixture UUIDs on PostgreSQL.
Fixes: #11524
Merge pull request #12016 from roderickvd/uuid_fixes

Auto-generate stable fixture UUIDs on PostgreSQL

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/fixtures.rb
	activerecord/test/cases/adapters/postgresql/uuid_test.rb
	activesupport/CHANGELOG.md
Merge branch 'master' into rm-uuid-fixtures

Conflicts:
	activerecord/CHANGELOG.md
	activesupport/CHANGELOG.md
No need to call send
Merge branch 'rm-uuid-fixtures'