Ignore certificate errors at higher level

We can do this on the OkHttpClient that is used by various clients so we
don't have to do it manually in every place we use it. This means we
don't convert https URLs to http to avoid certificate errors.
Store GuideInfo JSON for offline guides list

This vastly improves the performance of displaying the list of offline
guides by storing the GuideInfo JSON in addition to the full Guide and
using that to create the view.