Set database collation

Set database collation to match upstream settings. Related to https://github.com/gitlabhq/gitlabhq/pull/8195.
I think this should default to `nil`. The upstream template also does not set the collation: https://github.com/gitlabhq/gitlabhq/blob/a4e98f0ec985c91631f41c56317926f29365d95a/config/database.yml.postgresql#L4