Add a clear pin link in the list topics view

Meta: [Idea: Welcome message area on the home page for anonymous users](http://meta.discourse.org/t/idea-welcome-message-area-on-the-home-page-for-anonymous-users/6154/36)

As per @SamSaffron's suggestion, this adds a `clear pin` link in the list topics view ***once*** the user has read the whole topic.

It will also **replace**  the `read more` link when the excerpt is too long.

![image](https://f.cloud.github.com/assets/362783/456583/467bb27e-b370-11e2-99b9-b346402dcf5b.png)