Fixes `UndefinedError: 'settings' is undefined` problem.

**Problem**: `500.html` template extends the `base.html` and it requires `settings.APP_TITLE` value. `handler500` wasn't overridden with the `askbot.views.meta.server_error` and the `server_error` method didn't pass the settings to the template.

**Solution**: I modified the `urls.py` template to override the `handler500` value and extended the `server_error` method.

I also removed some unnecessary imports and `askbot.view.meta.page_not_found` (it is unused).

Related Pull Request is [164 - Fixed handler500 and handler404 in the project template, removed unused](https://github.com/ASKBOT/askbot-devel/pull/164) by @fitoria.

**Related questions**:

* [UndefinedError: 'settings' is undefined](http://askbot.org/en/question/9637/undefinederror-settings-is-undefined/)
* [settings is undefined](http://askbot.org/en/question/10427/settings-is-undefined/)