New thread tagging

[danielb]

In an effort to simplify searching on the site we have decided to implement a new feature that requires a tag be added when a new thread is made. When you create a new thread there is a field for entering relevant tags. If you leave this field empty the new feature will remind you to fill in some tags. Tags should be keywords that indicate what the thread is about. If the thread is about what guitars you like to play then the tags might be: "Ibanez, Gibson, favourite guitar".