URL tracking reimplementation

[Roman M. Yagodin]

• The DNN LinkClick.aspx have known issues, e.g. with the external URLs (see #92)
• DNN UrlTracking data is not related to the Document entity, but to the link
• Current implementation of URL tracking does not allow to integrate analytics for downloadable files and some external links.
• Tracking user and date/time of each click with DNN is probably overkill - I doubt it is widely used nor even useful (web analytics, again).

Solution:

• Add TrackClicks flag and number of clicks field directly to the Document entity, don't rely on DNN UrlTracking table anymore.
• Track clicks via JavaScript handler attached to the document links and WebAPI - this also simplify further integrations with web analytics engines like GA.
• This will resolve #80, #81, #92