Remove reply button on old comments

[int-y1]

fixes #823

• remove reply button when comment is too old and user doesn't have judge.change_comment.
• users can now reply to their own comment (previously, the backend allowed it, but the frontend didn't show the reply button)

[Riolku]

Do other people have opinions on this being a pure UI change?

[quantum5]

It looks to be a trivial change in the backend, so let's do it instead of letting people deliberately necropost to "show their 1337 h4x0r skills".

[int-y1]

It looks to be a trivial change in the backend, so let's do it instead of letting people deliberately necropost to "show their 1337 h4x0r skills".

done

[Riolku]

Overall looks fine.

[Riolku]

I might be the only one, but I feel like

if timezone.now() - parent_comment.time > settings.DMOJ_COMMENT_REPLY_TIMEFRAME and not self.request.user.has_perm('judge.change_comment'):

Is the more readable way of writing this.

[kiritofeng]

LGTM, thanks!

[Xyene]

LGTM, thanks!