Localization system for JSX-based content

[goldpbear]

As we move to React, a lot of translatable content is going to move from Handlebars templates to JSX. We'll need a way to localize this content. We'll also need to migrate existing translations to the new system. Migrating existing translations might need to be a manual step, but I don't think it will be too bad if we take it one set of components at a time.

I think we discussed the following library before and everyone seemed happy with it, so I'd propose we use it:
https://www.npmjs.com/package/i18next

Barring any objections, I'll go ahead and use this for #814 and other component ports.

[goldpbear]

As a side note, our overall translation system will likely straddle the new JSX-based system above and our existing gettext-based system until we're completely ported to React. If we do need to run a translation before we've ported everything, we (and the translator) will need to manage two translation files.

This isn't great, but I'd argue it's preferable to building a temporary tool that somehow knits the two translation systems together.