OSQA-823 - Show question list even for non-existing tags & prepopulate tags field for new questions

[cdman]

Add two new features:

• show (empty) question list even for non-existing tags (with a 418 return code to prevent indexing them)
• when the "ask question" link is clicked from a tag page the new question is prepopulated with the given tag

These features are useful when the OSQA instance is linked from an other site with some automatically generated tags (like "chapter1", "chapter2", ...).

[DanielThomas]

Nice idea, have no problem merging to my branch given it's configurable and I think I could actually do with this feature. As much I'm a fan of RFC 2324, that's not the correct way to prevent a page being indexed, you want a 200 to go to the browser. Can you change it to something standard please?

https://developers.google.com/webmasters/control-crawl-index/docs/robots_meta_tag

[cdman]

The current implementation returns the (customized) 404 page when a non-existing tag is requested. Ideally I would like to return a customized 404 page in this case, however (being a Python / Django n00b) I couldn't find a way to override the default 404 template.

In my opinion a custom error page is more in the spirit of the original implementation, is simpler to implement (doesn't depend on the templates containing a particular code) and ensures that most HTTP tools stop the processing of the page, not just well-written robots.