Added new template variables isdiscusspage, articleUrl and discussionUrl.

This makes it possible to treat discussion pages specially in the templates, and to have links between the article page and the discussion page.

I need this for a theme I'm working on, which doesn't use the "tabs". I believe as well that with this change, you could implement the "tabs" more or less entirely in the templates, without needing to hard-code them into the server-side logic. However, I didn't want to make such an invasive change myself, I want to leave that up to other people.

This has other uses as well, I'm not the first person to look for this feaure. See [here](https://groups.google.com/forum/#!msg/gitit-discuss/cRe3fl0IaLE/5Fese-7elVcJ) for example.