Add Japanese strings

Also including these changes:

1. The language code for the Japanese language is "ja", while the book uses "jp" everywhere. This discrepancy resulted in strings produced by moment.js not being localized.

    It would be better to use "ja" instead, but it would entail changing the repo name, submodule directory and all references to the string. Here I've simply added a `strings.moment_locale` attribute that takes precedence over `strings.lang` when loading the localization strings for moment.js.
2. `commit` and `scommit` helpers weren't using the `strings.commit` string
3. Other minor and apparent fixes