Issue title: Reader: hide follow button to match Calypso reader
 Issue body: As mentioned [here](https://github.com/wordpress-mobile/WordPress-Android/pull/4777#issuecomment-261280537), the Android app shows the Follow button in every stream but the Calypso reader doesn't. It should be removed from Followed Sites and a8c streams.
 Comments: 
 Comment 0: Addressed in #4834
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