Reader: hide replies initially for threads with lots of comments

[designsimply]

Raised by @bluefuton

@shaunandrews suggested that we hide comments initially when loading a thread with more than around 15 comments in it.

The user should see a link showing the number of replies (e.g. "3 replies"):

...along with a link in the open state to hide them again:

Originally suggested on 5842-gh-calypso-pre-oss