Issues regarding localization

[stefandesu]

A small issue collection in regards to localization:

• [x] mapping types are not localized (https://github.com/gbv/jskos-tools/issues/9)
• [x] editorial/scope notes need a proper German name
• [x] MappingBrowser: "show more" tooltip should have proper registry name
• [x] remove hardcoded .en or .de and replace with util.lmContent
• [x] adjust MappingsApp -> MappingsApp is deprecated anyway and will be replaced soon
• [x] ItemDetail title
• [x] move language selection to Settings (together with #173)
• [x] question: do we still need a language in the config file, and if yes what does it do?
• [x] question: do we need a separate fallbackLanguage config entry or do we use the language entry or do we use a hardcoded "en"?

Feel free to add more small changes to this.

[stefandesu]

I'm closing this even though editorial/scope notes don't have a proper German name. If there is one, please adjust it in config/locale.json.