Simplify tracking of URLs

Simplify tracking of URLs

In the prior commit, it needed to conceptually change the behavior of
`getTrackerURL($oldURL): $trackedURL`, but it danced around it (adding
more classes and similar-sounding-methods, but distributed elsewhere).

This re-consolidates to keep a cleaner separation of responsibilities
between `{Html,Text}ClickTracker.php` (*parsing an HTML or text message for URLS*)
and `TrackableURL.php` (*translating the URL*).