Clean HTML from RSS feeds with Jsoup and Show Link Warning

This adds HTML cleaning with the Jsoup library based on a whitelist. The resulting HTML is then used to create a `Spannable` in Android. This spannable is traversed and URLs are replaced by a custom dialog fragment that shows the following warning.

![device-2016-09-07-154936](/uploads/87a9800bde2676b0f2ddf5a894aa04eb/device-2016-09-07-154936.png)

Closes #569

See merge request !311