fix(legal): fix rendering of legal pages when loaded directly via url

I refactored the template fetching so that we can use it from the route handler when rendering pages dynamically and from the grunt task when generating pages statically.

Fixes #1372.

@shane-tomlinson r?
In line L104:R55 (https://github.com/mozilla/fxa-content-server/pull/1376/files#diff-957b1cc5354c61f5dec855f66a733381R55), does `getTemplate` need to be called with `DEFAULT_LANG`?