Show cached channel header on tvOS while channel loads

Render subscriber count, Subscribe button, and (for subscribed
channels) description in the tvOS loading state instead of just
avatar + name + spinner. Seed the in-memory author cache when
navigating to a channel from a video so the first-time channel
view has a name and avatar to display immediately.

Yattee/Services/Navigation/NavigationCoordinator.swift

@@ -333,6 +333,7 @@ final class NavigationCoordinator {
         } else if case .extracted = video.id.source, let authorURL = video.author.url {
             navigate(to: .externalChannel(authorURL))
         } else {
+            CachedChannelData.cacheAuthor(video.author)
             navigate(to: .channel(video.author.id, video.authorSource))
         }
     }