Create proper URL check for feature matrix entries

Create proper URL check for feature matrix entries

40d6891c had created a false illusion of working, which can
certainly be blamed on the patch author. That said, instead
of trying to work around what the standard Django filters
provide, it is more prudent to create a "URL checking"
function for the feature matrix descriptions. This can be
used both for the "featurelink" generation, as well as the
new check we make to get the stated goal of 40d6891c, i.e.
provide a clickable URL when that is the only content of a
feature description.