Cleanup the name fields naming and name normalization

[nyurik]

There are several issues with how we currently handle multilingual names. This is an umbrella ticket to track them all, and decide on a general strategy.

• Remove name_de and name_en (first mentioned in Is 'name_de' not a waste of space ? #769, implemented in [BREAKING] Remove name_en and name_de #928)
• Rename name:latin -> name_latin or name-Latn or ???
• Rename name:nonlatin -> name_nonlatin or ???
• Document the usage of name_int
• Handle OSM's Korean names - almost no name:ko_rm, a lot of name:ko-Latn - see Korean romanisation #537
• Handle OSM's Japanese names -- 100,000 of name:ja_rm, but they are deprecated per wiki in favor of name:ja-Latn (about 50,000) - was also mentioned in Korean romanisation #537

[systemed]

It would be good to have clarity on this, and for frequently used styles to be updated to match. Right now it's entirely unclear what third-party implementations of the OMT schema are meant to support - the spec says name, name_en and name_de, but styles appear to use name:latin or whatever.