CoinbaseController: return 200 response on webhook exception.

Coinbase will silently disable sending webhook notifications if it
receives too many 403/error responses.
Don't allow friend api operations on FakeSubreddits.
Use ModeratorMessageBuilder when viewing a modmail permalink.

Previously had been using UserMessageBuilder which does not include
replies when viewed by moderators.
Remove reddit-ie6-hax.css and reddit-ie7-hax.css from the Makefile since the source files were removed