Fix building without Crashlytics API key.

The app delegate checks whether CRASHLYTICS_API_KEY is defined at all, using
that to bypass a pointless init call to the Crashlytics code. The build script
needs to truncate the header file if there is no Crashlytics API key.
Posts view with unread posts hides "scroll to bottom" bar.

It's nice to show it by default when the posts are all read, so it's quick to
scroll to the bottom. But it's just in the way when there are unread posts.