Don't show the reply link when unable to reply due to depth limit

[abitmore]

Reported in #1093 :

Take this example : https://steemit.com/photography/@transisto/re-thecryptofiend-re-transisto-re-thecryptofiend-re-transisto-re-thecryptofiend-smoky-and-the-blanket-20170201t212614781z

I am able to click reply, write a long reply then get told "Comment is nested 6 posts deep, maximum depth is 6."

UI should know it's level 6 already since there is a depth field in comment_object, so better not show the "reply" link but show a message / tip.

Although after HF17 the limit will be higher, so this is less an issue, but it's low hanging fruit to improve UX.