Hide 'track thread' icon from free users in comments

[ninetyd]

We've had several requests from confused users who thought they weren't getting some notifications only to realize their account status didn't allow them to track comment threads. However, they can see the track icon and they can add the notification and nothing tells them it won't work. The icon at least should be hidden from them.

[ninetyd]

Here's one of the requests: http://www.dreamwidth.org/support/see_request?id=25671

[rahaeli]

Implementation note: I've looked into this a bit, and when you hit the bell "track this" icon on a comment, you only get the "email me when someone replies to this thread". (I thought it gave the option of both tracking the thread and tracking the comments on the whole post.)

So, it should be OK to just hide the bell icon when an account doesn't have access to track threads. (And it's probably best to add a "can_track_comment_threads" method in User.pm and check it that way, rather than directly checking paid status, if there isn't one already.)