Add lint:i18n to find missing & extra keys

[jacogr]

Add npm run lint:i18n reporting the missing keys (found in original, not in translation) as well as any extra keys (found in translation, not found in original)

*** Checking translations for de
  Missing account.button.delete
  Missing account.button.edit
  Missing account.button.password
...
  Missing walletSettings.rejected.busyStep.title
  Missing walletSettings.rejected.title
  Missing web.requestToken
Found 41 keys, missing 465 keys, 0 extraneous keys

*** Checking translations for nl
Found 506 keys, missing 0 keys, 0 extraneous keys

[ngotchac]

Looks good. Would it make sense to output it as a JSON though?

[jacogr]

I'll log an issue for that - honestly still unsure about exactly what would be useful here, so just started with identifying the gaps. (CSV could be useful as well)

[ngotchac]

Okay. Yes I guess JSON would be easy to read if we want to have some kind of metric integration (could display somewhere every languages with the percentage of translation on master)