Add image caching from the CDN
When we want to show logos or other images to the user, ideally we wouldn't be downloading the same image every time they reload the app (once every 12 hours) because @dhaynes3's AWS bill would skyrocket. We should instead download a copy of the XML manifest listing from the main page, download images that are needed, and store them in a Realm model.
We should then check, on update, whether the image information (notably last updated) matches what we have locally, and if so, display that instead of downloading a new image. If we do need to download an image, we should put it into that cache for later.
Finally, we need a "clear cache" button in Settings in case something gets messed up with the cache.