I18N manager supported customization translations

[ddivin-sc]

Meta-Info	Value
ExtProjectId	JCPROC-01
Original Estimation	8h
Remaining Estimation	8h

Description

We need to override translations on UI with custom translations. Found translation message should be changed by the following logic:

if (overriddenTranslations[key]) {
  return overriddenTranslations[key];
} else if (translations[locale][key]) {
  return translations[locale][key];
} else {
  return translations[fallbackLocale][key]
}

But the API will be changed like that:

const overriddenTranslations = {
    messages: {
      test: 'custom test message',
    }
}
let i18n = new I18nManager({
  locale, 
  fallbackLocale,
  localeFormattingInfo,
  overriddenTranslations
});

p.s. Don't forget updating tests

Original issue: https://github.com/OpusCapita/minsk-core-externals/issues/471
See also: https://github.com/OpusCapita/minsk-core-externals/issues/433

[ddivin-sc]

Please, review my changes in current repository and in OpusCapita/react-i18n#4