Add a live setting about allowing answering own questions.

A new live setting, ALLOW_ANSWER_OWN_QUESITONS, is defined. By default it's True, but it set to False, the user won't be able to answer their own questions.

The reason is that in some AskBot installations, the topic of discussion might be such that answering one's own question makes little sense.

In my very own case, I implemented this because the user would treat the AskBot site like a forum, and answer their own quesiton to continue the discussion.

It doesn't help that it says everywhere that that's not the correct way to use AskBot. :-)

git-diff has made a little mess in askbot/templates/question/new_answer_form.html, because the new {% if %} block alters the indentation of the code within, so it's hard to read, but it's truly quite simple instead.