FEATURE: add topic status namespace in RSS feed

Fixes [this bug report](https://meta.discourse.org/t/option-to-remove-pinned-topics-from-rss/10700?u=techapj).

Added `status` [namespace in RSS feed](http://www.disobey.com/detergent/2002/extendingrss2/).

Example:

```
<discourse:topicPinned>Yes</discourse:topicPinned>
<discourse:topicClosed>Yes</discourse:topicClosed>
<discourse:topicArchived>No</discourse:topicArchived>
```