Previously the `getMessage` function would throw if a substitution was
falsey. Now it will accept any substitution, including `undefined`.
A substitution of `null` or `undefined` will still be reported to
Sentry and printed to the console as an error, but it will not
interrupt execution. Any `null` or `undefined` substitutions will be
rendered as empty strings.
Ideally we'd never pass in `null` or `undefined` as a substitution, but
in practice this sometimes just occurs breifly between renders, which
isn't a severe enough problem to justify crashing the UI.
The detection of React component substitutions has been updated as
well, to ensure that `null` values aren't counted as React
substitutions.
* Update i18n-helper to allow substitutions of react components and wrapping of translation substrings
* Simplify code in i18n-helper.js related to substitutions, including react substitutions.
* Remove wrapper support from i18n in favour of using translations in substitutions.
* Fix i18n-helper substitution logic: ensure correct index of substitution is applied
* Throw error if there are not enough substitutions for a translation phrase
* Adds unit tests for now i18n-helper substitution functionality
* Fix grammar, react element line spacing and test layout+readability in i18n-helper.test.js
