feat: Translated TOS/PP text!

* Pull translated TOS/PP text from repo using bower, convert to HTML using grunt.
* Handle TOS/PP serving on the backend using new routes: `<locale>/legal/*`
* If `locale` is not specified or cannot be found, use a fallback template.
* Return 500 error with error message if a fallback template cannot be found.
* Front end requests localized TOS/PP partials from the backend, on demand, and inserts them into the DOM. partial vs full html rendering is determined by `Accepts` header.

Issue #634
feature(l10n): translate tos/pp
fix(client): Ensure the client specifies a language when requesting TOS/PP templates.
fix(server): Redirect with locale name that abide understands.

* Abide expects URLs to use `-` as the separator, we were redirecting to URLs with `_`.
fix(build): Normalize TOS/PP filenames to use the `_` separator.

* All other l10n files use the `_` separator, use them here too.
fix(tests): Ensure a real locale is used for TOS/PP template requests.
fix(build): Use lowercase extensions on output files in marked.
Merge pull request #674 from mozilla/issue-634-import-tos-pp

Localized TOS/PP!
fix(signup): choose what to sync checkbox should persist on the signup page