Fix issue #259. Allow links in the repository description

This change adds a DescriptionHtml() function to the Repository struct that returns the repositories description with links wrappend in <a> tags, that point to the given link in a new tab. It also updates the template for repositories to show the description with tags.

It doesn't really look at the links at this point, so there could be some problems with malicious links. This should probably not be merged at the moment, but just serve as a place to discuss this feature.