[WIP] l10n

Concept is ready for review.

Some things to note,

1. On the client side, we're translating the strings in backpack.js and then passing them to the template.
2. Need to decide on a naming convention for the messages.json file before we dive into adding all the template strings.
3. Given the difference in how we're going to handle client side templates and server side templates, we should probably separate them out somehow? So we know how to handle them case to case?

Thanks to @alicoding for making this possible!