Upgrade frontend-platform/i18n to react-intl 3.x

The react-intl library has a new maintainer (as of earlier this year) which has been doing a lot of cleanup and feature work. We're currently on v2.x, and upgrading to v3.x would give us some nice additions such as React hooks-based intl injection, and usage of intl outside of the context of react, which would be really helpful w/r/t making our code easier to reason.