[BUGFIX] Prevent superflous URL key regeneration

If a tinyurl record in the Backend is modified the URL key is now only
regenerated when a new record is created or when the URL has changed.
[FEATURE] Hit counter for URLs

If a URL is opened the hit will be counted in the tiny URL record.

The number of hits is displayed in the Backend.
[FEATURE] Comment field for URLs

It is now possible to add comments to URL records.

Thanks to Sebastian Lemke.