Add terms on client side.

I know that this should be proposed as a patch in mozilla-central.
I put it here to work on the patch and then I will make the appropriate patch to mozcentral.

 - [x] HTML / Ajax term loader based on the Accept Language of the browser (given back by loop-server)
 - [x] Extract ToS from legal-docs using grunt and bower.
Since this function looks very similar to the previous one on L18, not sure if you want to just make a helper function that would accept `lang` or `defaultLang` and do all the replacing and `$.get()` for you.

Also, not sure if we need to normalize the lang/locales a bit more. For example, what would happen if the config set `data.i18n.lang` to "en-us" (lowercase)? I assume we'd 404 on a case-sensitive OS and fail and try and load it again. I think the [i18n-abide](https://github.com/mozilla/i18n-abide/) lib has [`nomalizeLanguage()` and `normalizeLocale()` methods](https://github.com/mozilla/i18n-abide/blob/4f177fbb37c0fdab0a51582c6cf44a14685da758/lib/i18n.js#L376-L392) that handle that, although it's a Node module so you may have to copy/paste some code if this is browser only.